Patent Publication Number: US-8977794-B2

Title: Communication protocol system and method for a distributed-architecture heating, ventilation and air conditioning network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/167,135, filed by Grohman, et al., on Apr. 6, 2009, entitled “Comprehensive HVAC Control System” and U.S. Provisional Application Ser. No. 61/852,676, filed by Grohman, et al., on Apr. 7, 2009, and is also a continuation-in-part application of application Ser. No. 12/258,659, filed by Grohman on Oct. 27, 2008, entitled “Apparatus and Method for Controlling an Environmental Conditioning Unit,” all which are commonly assigned with this application and incorporated herein by reference. This application is also related to the following U.S. patent applications, which are filed on even date herewith, commonly assigned with this application and incorporated herein by reference: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Ser. No. 
                 Inventors 
                 Title 
               
               
                   
               
             
            
               
                 12/603,464 
                 Grohman, 
                 “Alarm and Diagnostics System and Method 
               
               
                   
                 et al. 
                 for a Distributed-Architecture Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning Network” 
               
               
                 12/603,534 
                 Wallaert, 
                 “Flush Wall Mount Control Unit and In- 
               
               
                   
                 et al. 
                 Set Mounting Plate for a Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning System” 
               
               
                 12/603,449 
                 Thorson, 
                 “System and Method of Use for a User 
               
               
                   
                 et al. 
                 Interface Dashboard of a Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning Network” 
               
               
                 12/603,526 
                 Grohman 
                 “Device Abstraction System and Method 
               
               
                   
                   
                 for a Distributed-Architecture Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning Network” 
               
               
                 12/603,526 
                 Grohman, 
                 “Communication Protocol System and 
               
               
                   
                 et al. 
                 Method for a Distributed-Architecture 
               
               
                   
                   
                 Heating, Ventilation and Air 
               
               
                   
                   
                 Conditioning Network” 
               
               
                 12/603,527 
                 Hadzidedic 
                 “Memory Recovery Scheme and Data 
               
               
                   
                   
                 Structure in a Heating, Ventilation and 
               
               
                   
                   
                 Air Conditioning Network” 
               
               
                 12/603,490 
                 Grohman 
                 “System Recovery in a Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning Network” 
               
               
                 12/603,473 
                 Grohman, 
                 “System and Method for Zoning a 
               
               
                   
                 et al. 
                 Distributed-Architecture Heating, 
               
               
                   
                   
                 Ventilation and Air Conditioning Network” 
               
               
                 12/603,525 
                 Grohman, 
                 “Method of Controlling Equipment in a 
               
               
                   
                 et al. 
                 Heating, Ventilation and Air 
               
               
                   
                   
                 Conditioning Network” 
               
               
                 12/603,512 
                 Grohman, 
                 “Programming and Configuration in a 
               
               
                   
                 et al. 
                 Heating, Ventilation and Air 
               
               
                   
                   
                 Conditioning Network” 
               
               
                 12/603,431 
                 Mirza, 
                 “General Control Techniques in a 
               
               
                   
                 et al. 
                 Heating, Ventilation and Air 
               
               
                   
                   
                 Conditioning Network” 
               
               
                   
               
            
           
         
       
     
    
    
     TECHNICAL FIELD 
     This application is directed, in general, to HVAC networks and, more specifically, to systems and methods for logical manipulation of system features. 
     BACKGROUND 
     Climate control systems, also referred to as HVAC systems (the two terms will be used herein interchangeably), are employed to regulate the temperature, humidity and air quality of premises, such as a residence, office, store, warehouse, vehicle, trailer, or commercial or entertainment venue. The most basic climate control systems either move air (typically by means of an air handler having a fan or blower), heat air (typically by means of a furnace) or cool air (typically by means of a compressor-driven refrigerant loop). A thermostat is typically included in a conventional climate control system to provide some level of automatic temperature and humidity control. In its simplest form, a thermostat turns the climate control system on or off as a function of a detected temperature. In a more complex form, the thermostat may take other factors, such as humidity or time, into consideration. Still, however, the operation of a thermostat remains turning the climate control system on or off in an attempt to maintain the temperature of the premises as close as possible to a desired set point temperature. Climate control systems as described above have been in wide use since the middle of the twentieth century and have, to date, generally provided adequate temperature management. 
     SUMMARY 
     A first aspect provides an HVAC data processing and communication network. In an embodiment, the network includes a first subnet controller and a system device. The subnet controller is coupled to a data bus and configured to arbitrate with a second subnet controller for control of the subnet. The system device is configured to transition from a reset state to a first state that includes pre-startup tasks, and transition from the first state to a second state that includes waiting for the subnet controller to provide configuration parameters to the system device. 
     Another aspect provides a method of manufacturing an HVAC data processing and communication network. In an embodiment, the method includes coupling a first subnet controller to a data bus and configuring a system device. The first subnet controller is configured to arbitrate with a second subnet controller for control of the subnet. The system device is configured to transition from a reset state to a first state that includes pre-startup tasks, and transition from the first state to a second state that includes waiting for the subnet controller to provide configuration parameters to the system device. 
    
    
     
       BRIEF DESCRIPTION 
       Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a high-level block diagram of an HVAC system according to various embodiments of the disclosure; 
         FIG. 2  is an embodiment of the disclosure of a network layer associated with the HVAC system; 
         FIG. 3  is a block diagram of a local controller of the disclosure; 
         FIG. 4  is a block diagram of a system device of the disclosure; 
         FIG. 5  illustrates an example grouping of devices on an RSBus subnet; 
         FIG. 6A  is an embodiment of an HVAC data processing and communication network having two subnets; 
         FIG. 6B  illustrates an embodiment of selectively isolating subnets; 
         FIG. 7  illustrates field definitions of an example message frame of the disclosure; 
         FIG. 8  illustrates an error frame of the disclosure; 
         FIG. 9  illustrates an embodiment of a Class 1 message format of the disclosure; 
         FIG. 10A  illustrates an embodiment of a Class 3 device status message format of the disclosure; 
         FIG. 10B  illustrates an embodiment of a Class 3 alarm message format of the disclosure; 
         FIG. 11  illustrates an embodiment of a Class 5 subnet controller message format of the disclosure; 
         FIG. 12  is a method of the disclosure illustrating startup of a local controller; 
         FIGS. 13A and 13B  are methods of the disclosure illustrating startup of a subnet controller; 
         FIG. 13C  is a state table an example state machine implementing a method of starting up a subnet controller; 
         FIG. 14  is a method of the disclosure illustrating an algorithm that may be employed by a subnet controller to assign Equipment Type to a device; 
         FIG. 15  is a method of the disclosure of conducting a dialog between a subnet controller and a demand unit according to the disclosure; and 
         FIGS. 16-24  illustrate various methods of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As stated above, conventional climate control systems have been in wide use since the middle of the twentieth century and have, to date, generally provided adequate temperature management. However, it has been realized that more sophisticated control and data acquisition and processing techniques may be developed and employed to improve the installation, operation and maintenance of climate control systems. 
     Described herein are various embodiments of an improved climate control, or HVAC, system in which at least multiple components thereof communicate with one another via a data bus. The communication allows identity, capability, status and operational data to be shared among the components. In some embodiments, the communication also allows commands to be given. As a result, the climate control system may be more flexible in terms of the number of different premises in which it may be installed, may be easier for an installer to install and configure, may be easier for a user to operate, may provide superior temperature and/or relative humidity (RH) control, may be more energy efficient, may be easier to diagnose, may require fewer, simpler repairs and may have a longer service life. 
       FIG. 1  is a high-level block diagram of a networked HVAC system, generally designated  100 . The HVAC system  100  may be referred to herein simply as “system  100 ” for brevity. In one embodiment, the system  100  is configured to provide ventilation and therefore includes one or more air handlers  110 . In an alternative embodiment, the ventilation includes one or more dampers  115  to control air flow through air ducts (not shown.) Such control may be used in various embodiments in which the system  100  is a zoned system. In an alternative embodiment, the system  100  is configured to provide heating and therefore includes one or more furnaces  120 , typically associated with the one or more air handlers  110 . In an alternative embodiment, the system  100  is configured to provide cooling and therefore includes one or more refrigerant evaporator coils  130 , typically associated with the one or more air handlers  110 . Such embodiment of the system  100  also includes one or more compressors  140  and associated condenser coils  142 , which are typically associated with one or more so-called “outdoor units”  144 . The one or more compressors  140  and associated condenser coils  142  are typically connected to an associated evaporator coil  130  by a refrigerant line  146 . In an alternative embodiment, the system  100  is configured to provide ventilation, heating and cooling, in which case the one or more air handlers  110 , furnaces  120  and evaporator coils  130  are associated with one or more “indoor units”  148 , e.g., basement or attic units that may also include an air handler. 
     For convenience in the following discussion, a demand unit  155  is representative of the various units exemplified by the air handler  110 , furnace  120 , and compressor  140 , and more generally includes an HVAC component that provides a service in response to control by the control unit  150 . The service may be, e.g., heating, cooling, humidification, dehumidification, or air circulation. A demand unit  155  may provide more than one service, and if so, one service may be a primary service, and another service may be an ancillary service. For example, for a heating unit that also circulates air, the primary service may be heating, and the ancillary service may be air circulation (e.g. by a blower). 
     The demand unit  155  may have a maximum service capacity associated therewith. For example, the furnace  120  may have a maximum heat output (often expressed in terms of British Thermal Units (BTU) or Joules), or a blower may have a maximum airflow capacity (often expressed in terms of cubic feet per minute (CFM) or cubic meters per minute (CMM)). In some cases, the demand unit  155  may be configured to provide a primary or ancillary service in staged portions. For example, blower may have two or more motor speeds, with a CFM value associated with each motor speed. 
     One or more control units  150  control one or more of the one or more air handlers  110 , the one or more furnaces  120  and/or the one or more compressors  140  to regulate the temperature of the premises, at least approximately. In various embodiments to be described, the one or more displays  170  provide additional functions such as operational, diagnostic and status message display and an attractive, visual interface that allows an installer, user or repairman to perform actions with respect to the system  100  more intuitively. Herein, the term “operator” will be used to refer collectively to any of the installer, the user and the repairman unless clarity is served by greater specificity. 
     One or more separate comfort sensors  160  may be associated with the one or more control units  150  and may also optionally be associated with one or more displays  170 . The one or more comfort sensors  160  provide environmental data, e.g. temperature and/or humidity, to the one or more control units  150 . An individual comfort sensor  160  may be physically located within a same enclosure or housing as the control unit  150 , in a manner analogous with a conventional HVAC thermostat. In such cases, the commonly housed comfort sensor  160  may be addressed independently. However, the one or more comfort sensors  160  may be located separately and physically remote from the one or more control units  150 . Also, an individual control unit  150  may be physically located within a same enclosure or housing as a display  170 , again analogously with a conventional HVAC thermostat. In such embodiments, the commonly housed control unit  150  and display  170  may each be addressed independently. However, one or more of the displays  170  may be located within the system  100  separately from and/or physically remote to the control units  150 . The one or more displays  170  may include a screen such as a liquid crystal or OLED display (not shown). 
     Although not shown in  FIG. 1 , the HVAC system  100  may include one or more heat pumps in lieu of or in addition to the one or more furnaces  120 , and one or more compressors  140 . One or more humidifiers or dehumidifiers may be employed to increase or decrease humidity. One or more dampers may be used to modulate air flow through ducts (not shown). Air cleaners and lights may be used to reduce air pollution. Air quality sensors may be used to determine overall air quality. 
     Finally, a data bus  180 , which in the illustrated embodiment is a serial bus, couples the one or more air handlers  110 , the one or more furnaces  120 , the one or more evaporator condenser coils  142  and compressors  140 , the one or more control units  150 , the one or more remote comfort sensors  160  and the one or more displays  170  such that data may be communicated therebetween or thereamong. As will be understood, the data bus  180  may be advantageously employed to convey one or more alarm messages or one or more diagnostic messages. All or some parts of the data bus  180  may be implemented as a wired or wireless network. 
     The data bus  180  in some embodiments is implemented using the Bosch CAN (Controller Area Network) specification, revision 2, and may be synonymously referred to herein as a residential serial bus (RSBus)  180 . The data bus  180  provides communication between or among the aforementioned elements of the network  200 . It should be understood that the use of the term “residential” is nonlimiting; the network  200  may be employed in any premises whatsoever, fixed or mobile. Other embodiments of the data bus  180  are also contemplated, including e.g., a wireless bus, as mentioned previously, and 2-, 3- or 4-wire networks, including IEEE-1394 (Firewire™, i.LINK™, Lynx™), Ethernet, Universal Serial Bus (e.g., USB 1.x, 2.x, 3.x), or similar standards. In wireless embodiments, the data bus  180  may be implemented, e.g., using Bluetooth™, Zibgee or a similar wireless standard. 
       FIG. 2  is a high-level block diagram of one embodiment of an HVAC data processing and communication network  200  that may be employed in the HVAC system  100  of  FIG. 1 . One or more air handler controllers (AHCs)  210  may be associated with the one or more air handlers  110  of  FIG. 1 . One or more integrated furnace controllers (IFCs)  220  may be associated with the one or more furnaces  120 . One or more damper controller modules  215 , also referred to herein as a zone controller module  215 , may be associated with the one or more dampers  115 . One or more unitary controllers  225  may be associated with one or more evaporator coils  130  and one or more condenser coils  142  and compressors  140  of  FIG. 1 . The network  200  includes an active subnet controller (aSC)  230   a  and an inactive subnet controller (iSC)  230   i . The aSC  230   a  may act as a network controller of the system  100 . The aSC  230   a  is responsible for configuring and monitoring the system  100  and for implementation of heating, cooling, humidification, dehumidification, air quality, ventilation or any other functional algorithms therein. Two or more aSCs  230   a  may also be employed to divide the network  200  into subnetworks, or subnets, simplifying network configuration, communication and control. Each subnet typically contains one indoor unit, one outdoor unit, a number of different accessories including humidifier, dehumidifier, electronic air cleaner, filter, etc., and a number of comfort sensors, subnet controllers and user interfaces. The iSC  230   i  is a subnet controller that does not actively control the network  200 . In some embodiments, the iSC  230   i  listens to all messages broadcast over the data bus  180 , and updates its internal memory to match that of the aSC  230   a . In this manner, the iSC  230   i  may backup parameters stored by the aSC  230   a , and may be used as an active subnet controller if the aSC  230   a  malfunctions. Typically there is only one aSC  230   a  in a subnet, but there may be multiple iSCs therein, or no iSC at all. Herein, where the distinction between an active or a passive SC is not germane the subnet controller is referred to generally as an SC  230 . 
     A user interface (UI)  240  provides a means by which an operator may communicate with the remainder of the network  200 . In an alternative embodiment, a user interface/gateway (UI/G)  250  provides a means by which a remote operator or remote equipment may communicate with the remainder of the network  200 . Such a remote operator or equipment is referred to generally as a remote entity. A comfort sensor interface  260 , referred to herein interchangeably as a comfort sensor (CS)  260 , may provide an interface between the data bus  180  and each of the one or more comfort sensors  160 . The comfort sensor  260  may provide the aSC  230   a  with current information about environmental conditions inside of the conditioned space, such as temperature, humidity and air quality. 
     For ease of description, any of the networked components of the HVAC system  100 , e.g., the air handler  110 , the damper  115 , the furnace  120 , the outdoor unit  144 , the control unit  150 , the comfort sensor  160 , the display  170 , may be described in the following discussion as having a local controller  290 . The local controller  290  may be configured to provide a physical interface to the data bus  180  and to provide various functionality related to network communication. The SC  230  may be regarded as a special case of the local controller  290 , in which the SC  230  has additional functionality enabling it to control operation of the various networked components, to manage aspects of communication among the networked components, or to arbitrate conflicting requests for network services among these components. While the local controller  290  is illustrated as a stand-alone networked entity in  FIG. 2 , it is typically physically associated with one of the networked components illustrated in  FIG. 1 . 
       FIG. 3  illustrates a high-level block diagram of the local controller  290 . The local controller  290  includes a physical layer interface (PLI)  310 , a non-volatile memory (NVM)  320 , a RAM  330 , a communication module  340  and a functional block  350  that may be specific to the demand unit  155 , e.g., with which the local controller  290  is associated. The PLI  310  provides an interface between a data network, e.g., the data bus  180 , and the remaining components of the local controller  290 . The communication module  340  is configured to broadcast and receive messages over the data network via the PLI  310 . The functional block  350  may include one or more of various components, including without limitation a microprocessor, a state machine, volatile and nonvolatile memory, a power transistor, a monochrome or color display, a touch panel, a button, a keypad and a backup battery. The local controller  290  may be associated with a demand unit  155 , and may provide control thereof via the functional block  350 , e.g. The NVM  320  provides local persistent storage of certain data, such as various configuration parameters, as described further below. The RAM  330  may provide local storage of values that do not need to be retained when the local controller  290  is disconnected from power, such as results from calculations performed by control algorithms. Use of the RAM  330  advantageously reduces use of the NVM cells that may degrade with write cycles. 
     In some embodiments, the data bus  180  is implemented over a 4-wire cable, in which the individual conductors are assigned as follows: 
     R—the “hot”—a voltage source, 24 VAC, e.g. 
     C—the “common”—a return to the voltage source. 
     i+—RSBus High connection. 
     i−—RSBus Low connection. 
     The disclosure recognizes that various innovative system management solutions are needed to implement a flexible, distributed-architecture HVAC system, such as the system  100 . More specifically, cooperative operation of devices in the system  100 , such as the air handler  110 , outdoor unit  144 , or UI  240  is improved by various embodiments presented herein. More specifically still, embodiments are presented of communications protocols among networked HVAC devices that provide a robust means of communicating within an installation site, and simplified configuration of the system relative to conventional systems. 
       FIG. 4  illustrates a device  410  according to the disclosure. The following description pertains to the HVAC data processing and communication network  200  that is made up of a number of system devices  410  operating cooperatively to provide HVAC functions. Herein after the system device  410  is referred to more briefly as the device  410  without any loss of generality. The term “device” applies to any component of the system  100  that is configured to communicate with other components of the system  100  over a wired or wireless network. Thus, the device  410  may be, e.g., the air handler  110  in combination with its AHC  210 , or the furnace  120  in combination with its IFC  220 . This discussion may refer to a generic device  410  or to a device  410  with a specific recited function as appropriate. An appropriate signaling protocol may be used to govern communication of one device with another device. While the function of various devices  410  in the network  200  may differ, each device  410  shares a common architecture for interfacing with other devices, e.g. the local controller  290  appropriately configured for the HVAC component  420  with which the local controller  290  is associated. The microprocessor or state machine in the functional block  350  may operate to perform any task for which the device  410  is responsible, including, without limitation, sending and responding to messages via the data bus  180 , controlling a motor or actuator, or performing calculations. A system status display  430  is described below. 
     In various embodiments, signaling between devices  410  relies on messages. Messages are data strings that convey information from one device  410  to another device  410 . The purpose of various substrings or bits in the messages may vary depending on the context of the message. Generally, specifics regarding message protocols are beyond the scope of the present description. However, aspects of messages and messaging are described when needed to provide context for the various embodiments described herein. 
       FIG. 5  illustrates an embodiment of the disclosure of a network of the disclosure generally designated  500 . The network  500  includes an aSC  510 , a user interface  520 , a comfort sensor  530  and a furnace  540  configured to communicate over a data bus  550 . In some embodiments these devices form a minimum HVAC network. In addition, the network  500  is illustrated as including an outdoor unit  560 , an outdoor sensor  570 , and a gateway  580 . The furnace  540  and outdoor unit  560  are provided by way of example only and not limited to any particular demand units. The aSC  510  is configured to control the furnace  540  and the outdoor unit  560  using, e.g., command messages sent via the data bus  550 . The aSC  510  receives environmental data, e.g. temperature and/or humidity, from the comfort sensor  530 , the furnace  540 , the outdoor sensor  570  and the outdoor unit  560 . The data may be transmitted over the data bus  550  by way of messages formatted for this purpose. The user interface  520  may include a display and input means to communicate information to, and accept input from, an operator of the network  500 . The display and input means may be, e.g., a touch-sensitive display screen, though embodiments of the disclosure are not limited to any particular method of display and input. 
     The aSC  510 , comfort sensor  530  and user interface  520  may optionally be physically located within a control unit  590 . The control unit  590  provides a convenient terminal to the operator to effect operator control of the system  100 . In this sense, the control unit is similar to the thermostat used in conventional HVAC systems. However, the control unit  590  may only include the user interface  520 , with the aSC  510  and comfort sensor  530  remotely located from the control unit  590 . 
     As described previously, the aSC  510  may control HVAC functionality, store configurations, and assign addresses during system auto configuration. The user interface  520  provides a communication interface to provide information to and receive commands from a user. The comfort sensor  530  may measure one or more environmental attributes that affect user comfort, e.g., ambient temperature, RH and pressure. The three logical devices  510 ,  520 ,  530  each send and receive messages over the data bus  550  to other devices attached thereto, and have their own addresses on the network  500 . In many cases, this design feature facilitates future system expansion and allows for seamless addition of multiple sensors or user interfaces on the same subnet. The aSC  510  may be upgraded, e.g., via a firmware revision. The aSC  510  may also be configured to release control of the network  500  and effectively switch off should another SC present on the data bus  550  request it. 
     Configuring the control unit  590  as logical blocks advantageously provides flexibility in the configuration of the network  500 . System control functions provided by a subnet controller may be placed in any desired device, in this example the control unit  590 . The location of these functions therein need not affect other aspects of the network  500 . This abstraction provides for seamless upgrades to the network  500  and ensures a high degree of backward compatibility of the system devices  410  present in the network. The approach provides for centralized control of the system, without sacrificing flexibility or incurring large system upgrade costs. 
     For example, the use of the logical aSC  510  provides a flexible means of including control units on a same network in a same conditioned space. The system, e.g., the system  100 , may be easily expanded. The system retains backward compatibility, meaning the network  500  may be updated with a completely new type of equipment without the need to reconfigure the system, other than substituting a new control unit  590 , e.g. Moreover, the functions provided by the subnet controller may be logically placed in any physical device, not just the control unit  590 . Thus, the manufacturer has greater flexibility in selecting devices, e.g., control units or UIs, from various suppliers. 
     In various embodiments, each individual subnet, e.g., the network  500 , is configured to be wired as a star network, with all connections to the local controller  290  tied at the furnace  120  or the air handler  110 . Thus, each indoor unit, e.g., the furnace  120 , may include three separate connectors configured to accept a connection to the data bus  180 . Two connectors may be 4-pin connectors: one 4-pin connector may be dedicated for connecting to an outdoor unit, and one may be used to connect to equipment other than the outdoor unit. The third connector may be a 2-pin connector configured to connect the subnet of which the indoor unit is a member to other subnets via the i+/i− signals. As described previously, a 24 VAC transformer associated with the furnace  120  or air handler  110  may provide power to the system devices  410  within the local subnet via, e.g., the R and C lines. The C line may be locally grounded. 
       FIG. 6A  illustrates a detailed connection diagram of components of a network  600 A according to one embodiment of the disclosure. The network  600 A includes a zone  605  and a zone  610 . The zones  605 ,  610  are illustrated without limitation as being configured as subnets  615 ,  620 , respectively. The subnet  615  includes an air conditioning (AC) unit  630 , a UI/G  640 , an outdoor sensor (OS)  650 , a control unit  660 , and a furnace  670 . The control unit  660  includes an SC  662 , a UI  664  and a comfort sensor  666 , each of which is independently addressable via a data bus  180   a . The subnet  620  includes a control unit  680 , a heat pump  690  and a furnace  695 . The control unit  680  houses an SC  682 , a UI  684  and a comfort sensor  686 , each of which is independently addressable via a data bus  180   b . In various embodiments and in the illustrated embodiment each individual subnet, e.g., the subnets  615 ,  620  are each configured to be wired as a star network, with connections to all devices therein made at a furnace or air handler associated with that subnet. Thus, e.g., each of the devices  630 ,  640 ,  650 ,  660  is connected to the data bus  180   a  at the furnace  670 . Similarly, each device  680 ,  690  is connected to the subnet  620  at the furnace  695 . Each furnace  670 ,  695 , generally representative of the indoor unit  148 , may include a connection block configured to accept a connection to the RSBus  180 . For example, two terminals of the connection block may be 4-pin connectors. In one embodiment, one 4-pin connector is dedicated to connecting to an outdoor unit, for example the connection from the furnace  670  to the AC unit  630 . Another 4-pin connector is used to connect to equipment other than the outdoor unit, e.g., from the furnace  670  to the UI/G  640 , the OS  650 , and the control unit  660 . A third connector may be a 2-pin connector configured to connect one subnet to another subnet. In the network  600 A, e.g., the subnet  615  is connected to the subnet  620  via a wire pair  698  that carries the i+/i− signals of the serial bus. As described previously with respect to the furnace  120 , a transformer located at the furnace  670  may provide power to the various components of the subnet  615 , and a transformer located at the furnace  695  may provide power to the various components of the subnet  620  via R and C lines. As illustrated, the C line may be locally grounded. 
     This approach differs from conventional practice, in which sometimes a master controller has the ability to see or send commands to multiple controllers in a single location, e.g., a house. Instead, in embodiments of which  FIG. 6A  is representative there is no master controller. Any controller (e.g. the SCs  662 ,  682 ) may communicate with any device, including other controllers, to make changes, read data, etc. Thus, e.g., a user located on a first floor of a residence zoned by floor may monitor and control the state of a zone conditioning a second floor of the residence without having to travel to the thermostat located on the second floor. This provides a significant convenience to the user, who may be a resident, installer or technician. 
       FIG. 7  illustrates an example embodiment of a message frame generally designated  700 . The message frame  700  is configurable to send messages between one local controller  290  and another local controller  290 , e.g., between the UI  240  and the AHC  210 . It is to be understood that the message frame  700  is but one of several possible schemes to communicate between local controllers  290 . Those of skill in the pertinent arts will recognize that other equivalent schemes are within the scope of the disclosure. 
     Messages may be communicated in a manner compatible with a two-wire bus architecture. In some cases, a controller-area network is an appropriate communication standard. In an example embodiment, messages follow a format based on the Bosch CAN2.0B (hereinafter “CAN”) standard. The following aspects of the CAN standard are described by way of example, with no implied limitation on messaging formats otherwise within the scope of the disclosure. 
     As will be appreciated by those skilled in the pertinent art, the bus in the CAN standard can have one of two complementary logical values: “dominant” or “recessive”. During simultaneous transmission of dominant and recessive bits, the resulting bus value will be dominant. For example, in case of a wired-AND implementation of the bus, the dominant level would be represented by a logical 0 and the recessive level by a logical 1. In this context a dominant bit is a bit that “wins” when a dominant and a recessive bit are simultaneously asserted on the CAN bus. 
     As illustrated in  FIG. 7 , a single message frame may include a Start of Frame (SOF) bit  710 , an Arbitration Field (AF)  720 , a Control Field (CF)  730 , a Data Field (DF)  740 , a CRC Field  750 , an ACK Field  760  and an End of Frame (EOF) Field  770 . 
     Each message frame starts with a dominant SOF bit  710 , e.g., a logical 0. At least some of the local controllers  290  on the network  200  that are ready to transmit messages synchronize to the SOF bit  710  generated by the local controller  290  that initializes the transmission. In some cases, the SC  230  performs the initialization. This aspect is discussed in greater detail below. It may be preferable in some cases that all of the local controllers  290  on the network  200  synchronize in this manner. 
     The AF  720  may include a number of bits as identifier (ID) bits. The illustrated embodiment includes two subfields. A first subfield  722  includes, e.g., 11 base ID bits, while a second subfield  724  includes, e.g., 18 extended ID bits. This configuration is an example of a CAN extended format. Those skilled in the pertinent art will appreciate that in other embodiments, a standard format message frame  700  may be used. An SRR bit and an IDE bit separate the first subfield  722  and the second subfield  724 , and a RTR bit ends the AF  720 . In some embodiments the SRR bit and IDE bit are always set to 1 and the RTR bit is always set to 0. 
     In the message frame  700  the CF  730  is illustrated as including, e.g., two reserved bits R 0  and R 1  and a 4-bit Data Length Code (DLC) Field. The reserve bits are always sent as recessive, but the receivers should accept them without any errors regardless if they are recessive or dominant. The DLC Field determines the number of bytes in the DF  740 . 
     The DF  740  may range from 0 to 64 bits. The case of 0 bits, of course, represents the special case that no data is send by the message frame  700 . In all but this special case, data may be segmented into multiples of 8 bits (bytes) with maximum of 8 bytes. 
     The CRC Field  750  contains a checksum calculated on the SOF bit  710 , the AF  720 , CF  730  and DF  740 . The CRC field  750  is illustrated in this example embodiment of the CAN standard as being 15 bits wide. Of course other CRC widths may be used where appropriate for other communication standards. The computation of the CRC may be determined as per the CAN2.0 standard, e.g. The CRC field  750  is terminated in a suitable manner, e.g., by a Delimiter Bit that is always recessive. 
     The ACK field  760  is two bits long and contains an ACK SLOT (ACK) and an ACK delimiter (Del). A transmitting local controller  290 , e.g., the SC  230 , sends two recessive bits. A receiving local controller  290 , e.g., the AHC  210 , reports the correct receipt of a message to the transmitting local controller  290  by asserting a dominant bit during the ACK slot. Thus the transmitting local controller  290  can detect that another local controller  290  is present on the network to receive the message. However, the acknowledgement by the receiving local controller  290  does not confirm the validity of the message data. 
     The EOF field  770  is delimited by a flag sequence of seven consecutive recessive bits. 
     The CAN standard prohibits the occurrence of more than five consecutive bits of a same value in the SOF bit  710 , the AF  720 , the CF  730 , the DF  740 , and the CRC field  750 . Whenever a transmitting local controller  290  detects five consecutive bits of identical value in the bit stream to be transmitted it automatically inserts a complementary bit in the actual transmitted bit stream. 
     The CAN standard defines five types of errors that are not mutually exclusive: 
     Bit Error—while sending any bits on the bus, the transmitting local controller  290  also monitors the bus. When the state of the bus is detected to be different from the intended state, a bit error normally occurs. Exceptions to this general case include when a recessive bit is sent in an AF  720  and a dominant bit is read back. This event signifies a case of lost arbitration rather than a bit error. The ACK field  760  is sent as a recessive bit. When at least one other active local controller  290  is present on the bus, in routine operation the local controller  290  sets the field to the dominant state. Note that a local controller  290  sending a Passive Error Flag and detecting a dominant bit does not interpret this as a Bit Error. A bit error may indicate in some circumstances a collision between a message the local controller  290  is attempting to publish to the data bus  180  and a message published to the data bus  180  by another local controller. 
     Bit Stuffing Error: this error occurs when a 6th consecutive equal bit level is detected in the message field comprising the SOF bit  710 , the AF  720 , the CF  730 , the DF  740  and the CRC field  750 . 
     CRC Error: each receiving local controller  290  calculates the CRC in the same manner as the transmitting local controller  290 . The CRC error is generated when the calculated value is different from the value received on the RSBus bus  180 . 
     Form Error: this error occurs when a fixed-form bit field (a delimiter, EOF Field or inter-frame space) contains one or more illegal bits. For the receiving local controller  290 , a dominant bit received in the EOF bit should not be considered an error. 
     Acknowledgment Error: this error represents the condition that the transmitting local controller  290  determines that no receiving local controller  290  has asserted a dominant bit during the ACK transmission as described above. 
       FIG. 8  illustrates an embodiment of an error frame, generally designated  800 . The RSBus  180  may provide active and passive error frames in conformity with the CAN standard. The error frame  800  includes an error flag field  810  and an error delimiter  820 . The error flag field  810  may be superimposed. In an active error frame, the superposed flags are dominant, whereas in a passive error frame, the flags are recessive. 
     The majority of transmission errors may be addressed by retransmitting the message according to the CAN2.0 standard. More specifically, each error type listed above may be handled as follows: 
     Bit Error: An Error Frame may be generated which starts with the next bit-time. 
     Bit Stuffing Error: A node that detects a violation of bit stuffing (e.g., more than 5 bits of the same state) may generate an Error Frame, which causes the sending local controller  290  to resend the message. 
     For Class 5 messages that include the 5-bit Order Number, the position of the Order Number in the message ID may be shifted left by one position so as to prevent interference with the position of the Transport Protocol bit C5MID0/TP. The Order Number of the SC  230  may also be a number calculated from the number of other SCs detected on the subnet. 
     Form Error: A sending local controller  290  that detects a dominant bit in the Delimiter, End of Frame (EOF) field or Inter-frame Space may generate an Active Error Frame. The sending local controller  290  may resend the message in response to the Active Error Frame. 
     Acknowledge Error: At least one receiving local controller  290  is expected to set the acknowledge bit to dominant after the message is sent by the transmitting local controller  290 . If the acknowledge bit is not set to dominant, the sending local controller  290  may resend the message. 
     Under certain conditions, a local controller  290  may be placed in a fault confinement condition to limit the operation thereof. Each local controller  290  keeps a count of detected transmit and receive errors. Under some conditions, the local controller  290  may enter one of three error states: error active, error passive, and bus off. 
     A local controller  290  is normally in the error active state. In this state the local controller  290  can interrupt a current message in progress by signaling an error via an active error frame  800 . The transmitting local controller  290  detects the active error frame  800  and resends the message as described above. Each local controller  290  may keep a separate count of transmit errors and receive errors. The local controller  290  remains in the error active state until an error count exceeds a lower limit value. The limit value 127 may be chosen for convenience, but any desired number may be used. In some embodiments, the error value is 2 n −1, n being an integer. 
     A local controller  290  enters the error passive state when either the transmit or receive error count exceeds 127. In the event that one of the error counts exceeds 127, the local controller  290  may generate an alarm condition alerting the SC  230  to the error state. An alarm condition may be signified by a DEVICE Communications Problem alarm. The alarm may be cleared when the local controller  290  enters the error active state. In the error passive state, a local controller  290  is configured to refrain from interrupting a message in progress. The local controller  290  may, however, generate passive error frames  800 . 
     The local controller  290  enters the bus off state when the transmit error count exceeds an upper limit value. In some embodiments, the upper limit value is 2 n+1 −1, where n is the integer selected for the lower limit value described above. Thus, in one example, if the lower limit value is 127, the upper limit value may be 255. 
     When the error count exceeds the upper limit value, the affected local controller  290  is configured to refrain from sending messages on the RSBus  180 . However, the local controller  290  may continue to monitor activity on the RSBus  180 . A local controller  290  which is in bus off state may enter the error active state after a reset. The device reset condition may be the expiration of a timer that starts upon the local controller  290  entering the bus-off state. In an example embodiment, the timer expires after 5 minutes. When the local controller  290  is reset by any means, the local controller  290  may reset its transmit error count. 
     Referring back to  FIG. 7 , each message frame  700  may be limited in the amount of data that may be sent thereby. When implemented using the CAN2.0 standard, for example, the DF  740  can contain a maximum of eight bytes. In some cases, it may be desirable to send more than eight bytes of data from one local controller  290  to another local controller  290 . In such cases, the sending local controller  290  may send a message longer than eight bytes by partitioning the message into multiple message frames  700 . A mechanism referred to as “transport protocol” is provided by some embodiments to enable sending such messages. In some embodiments, this mechanism is based on the ISO/DIS Standard 15765-2, incorporated herein by reference as if reproduced in its entirety. Herein after, this standard is referred to as the “15765-2 standard” for brevity. The 15765-2 standard provides for message sequences that include up to 4095 bytes. 
     The 15765-2 standard uses the addressing format as described below with respect to the message addressing scheme. Thus transport protocol messages may follow the same format as other messages broadcast over the RSBus  180 . However, transport protocol messages may be distinguished from non-transport protocol messages at the appropriate layer of the protocol stack based on the ID of the message in question. 
     Referring to  FIG. 7 , the DF  740  may include from 0 to eight bytes of data, where each byte comprises 8 bits. In some cases, a local controller  290  may need to convey more than eight bytes of data to another local controller  290 . 
     In various embodiments the local controllers  290  are configured to implement full-duplex transport protocol communication. Such communication is defined, e.g., in Section 6.7.3 of the 15765-2 standard. All local controllers  290 , except the SC  230  and the UI/G  250 , are single session transfer protocol devices. The SC  230  and the UI/G  250  support up to 4 concurrent transport protocol sessions. When single session devices are engaged in a transport protocol receive session, they are not required to respond to a new transport protocol receive session request. 
     In such cases the SC  230  and the UI/G  250  may ignore incoming first frames. The transmitting local controller  290  may then retry sending the first frame a number of times. In some embodiments, the requesting local controller  290  retries twice, each time after a one-second timeout. If three consecutive attempts fail, the local controller  290  may issue an alarm signifying that the receiving local controller  290  is unresponsive and may abort the communication attempt. Analogously, the same single-session transport protocol device will not request another transport protocol send session unless the currently ongoing send session is completed. In some embodiments, all single frame transport protocol messages are sent and received regardless of the state of the multi-frame send or receive sessions. 
     In some embodiments, a transport protocol block size is eight, and a separation time may be 5 ms. However, the local controllers  290  may be configured to use other values, or to override default values, when necessary for effective communication. 
     In some cases, one or more errors may be encountered during a transfer protocol session. Various embodiments provide error handling consistent with those described by the 15765-2 standard. 
     Each logical local controller  290  on the RSBus  180  may be identified by an Equipment Type (ET) number. The Equipment Type number serves as an identifier of a class of logical local controllers  290 . In some cases, there may be multiple Equipment Type numbers for a same device class. Table I below lists an example embodiment of Equipment Type numbers for various classes of equipment. The values presented in Table I apply to this example embodiment, and are provided without limitation for illustration purposes. Those skilled in the pertinent art will recognize that various equivalents may be implemented within the scope of the disclosure. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 RSBus Equipment Types 
               
            
           
           
               
               
               
               
            
               
                   
                 Number 
                 Equipment Type Number (in binary form) 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Equipment Type 
                 Range 
                 Bit 8 
                 Bit 7 
                 Bit 6 
                 Bit 5 
                 Bit 4 
                 Bit 3 
                 Bit 2 
                 Bit 1 
                 Bit 0 
                 Comments 
               
               
                   
               
               
                 Subnet Controllers 
                 0h-Fh 
                 0 
                 0 
                 0 
                 0 
                 0 
                 x 
                 x 
                 x 
                 x 
                 Active Subnet 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Controller = 0h 
               
               
                 Furnace 
                 10h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                 Air Handler 
                 11h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
                 1 
               
               
                 Air Conditioner 
                 12h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 1 
                 0 
               
               
                 Heat Pump 
                 13h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 0 
                 1 
                 1 
               
               
                 Humidifier 
                 14h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 1 
                 0 
                 0 
               
               
                 Dehumidifier 
                 15h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 1 
                 0 
                 1 
               
               
                 Damper Control Modules 
                 16h-17h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 0 
                 1 
                 1 
                 X 
               
               
                 ERV 
                 18h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 0 
                 0 
               
               
                 HRV 
                 19h 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 0 
                 1 
               
               
                 Dual Fuel Module 
                 1Ah 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 1 
                 0 
               
               
                 UV Light 
                 1Bh 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 1 
                 1 
               
               
                 Media Air Cleaner 
                 1Ch 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 0 
                 0 
               
               
                 Electronic Air Cleaner 
                 1Dh 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 0 
                 1 
               
               
                 IAQ Analyzer 
                 1Eh 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 0 
               
               
                 Twinning Module 
                 1Fh 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Wireless Comfort 
                 20h-3Fh 
                 0 
                 0 
                 0 
                 1 
                 x 
                 x 
                 x 
                 x 
                 x 
                 Wireless Gateways may 
               
               
                 Sensors 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 be configured at 20h 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 or at 30h. Individual 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 sensors may then be 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 added from that 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 address on until a 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 maximum number is 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 reached, e.g., 32. 
               
               
                 Comfort Sensors 
                 40h-4Fh 
                 0 
                 0 
                 1 
                 0 
                 0 
                 x 
                 x 
                 x 
                 x 
               
               
                 Wireless Outdoor 
                 50h-5Fh 
                 0 
                 0 
                 1 
                 0 
                 1 
                 x 
                 x 
                 x 
                 x 
                 A Wireless Gateway may 
               
               
                 Sensor 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 be configured at 50h. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Individual addresses 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 are added to this base 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 address to a maximum 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 number of sensors, 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 e.g., 16. 
               
               
                 Outdoor Sensors 
                 60h-63h 
                 0 
                 0 
                 1 
                 1 
                 0 
                 0 
                 0 
                 x 
                 x 
                 The number of sensors 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 on each subnet may be 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 limited, e.g., to 4. 
               
               
                 Not Used 
                 64h-6Fh 
                 0 
                 0 
                 1 
                 1 
                 0 
                 ? 
                 ? 
                 ? 
                 ? 
                 Expansion 
               
               
                 User Interfaces/ 
                 70h-7Fh 
                 0 
                 0 
                 1 
                 1 
                 1 
                 x 
                 x 
                 x 
                 x 
                 User Interfaces are 70h- 
               
               
                 Gateways 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 7Bh. Gateways are 7Ch-7Fh 
               
               
                 Not Used 
                 80h-1DFh 
                 1 
                 ? 
                 ? 
                 ? 
                 ? 
                 ? 
                 ? 
                 ? 
                 ? 
                 Expansion 
               
               
                 Reserved 
                 1E0h-1FF 
                 1 
                 1 
                 1 
                 1 
                 x 
                 x 
                 x 
                 x 
                 x 
                 Reserved for NVM Flashing 
               
               
                   
               
            
           
         
       
     
     In various embodiments, a local controller  290  may be configured to notify the aSC  230   a  that it cannot be configured as commanded. The notification may take the form of an appropriately configured message sequence from the local controller  290  to the aSC  230   a.    
     Table II below illustrates an embodiment of a message addressing scheme of the disclosure. A message ID of this embodiment includes 29 bits, providing a pool of more than 5E8 different messages. This message pool is divided into eight message classes identified by the three most significant bits of the Message ID, bits  26 - 28 . Each message class may be designated for a different purpose, as indicate. In the illustrated embodiment, classes 0, 2, 4 and 7 are not currently defined. 
     
       
         
           
               
             
               
                 TABLE II 
               
               
                   
               
               
                 RSBus Message Classes 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Message 
                 CAN extended 29-bit message ID 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Class 
                 28 
                 27 
                 26 
                 25 
                 24 
                 23 
                 22 
                 21 
                 20 
                 19 
               
               
                   
               
               
                 Class 0 
                 0 
                 0 
                 0 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Class 1 
                 0 
                 0 
                 1 
                 C1MID8 
                 C1MID7 
                 C1MID6 
                 C1MID5 
                 C1MID4 
                 C1MID3 
                 C1MID2 
               
               
                 UI/G 
               
               
                 messages 
               
               
                 Class 2 
                 0 
                 1 
                 0 
               
               
                 Class 3 
                 0 
                 1 
                 1 
                 AL 
                 C3MID12/PR0 
                 C3MID11/PR1 
                 C3MID10/S/C 
                 C3MID9 
                 C3MID8 
                 C3MID7 
               
               
                 Broadcast 
               
               
                 messages 
               
               
                 Class 4 
                 1 
                 0 
                 0 
               
               
                 Class 5 
                 1 
                 0 
                 1 
                 C5MID12 
                 C5MID11 
                 C5MID10 
                 C5MID9 
                 C5MID8 
                 C5MID7 
                 C5MID6 
               
               
                 SC 
               
               
                 messages 
               
               
                 Class 6 
                 1 
                 1 
                 0 
                 C6MID9 
                 C6MID8 
                 C6MID7 
                 C6MID6 
                 C6MID5 
                 C6MID4 
                 C6MID3 
               
               
                 Diagnostic 
               
               
                 messages 
               
               
                 Class 7 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
           
               
               
            
               
                 Message 
                 CAN extended 29-bit message ID 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Class 
                 18 
                 17 
                 16 
                 15 
                 14 
                 13 
                 12 
                 11 
               
               
                   
               
               
                 Class 0 
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Class 1 
                 C1MID1 
                 C1MID0/TP 
                 Destination 
               
               
                 UI/G 
                   
                   
                 or Source 
               
               
                 messages 
                   
                   
                 Equipment Type 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Class 2 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Class 3 
                 C3MID6 
                 C3MID5 
                 C3MID4 
                 C3MID3 
                 C3MID2 
                 C3MID1 
                 C3MID0 
                 AS 
               
               
                 Broadcast 
               
               
                 messages 
               
               
                 Class 4 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Class 5 
                 C5MID5 
                 C5MID4 
                 C5MID3 
                 C5MID2 
                 C5MID1 
                 C5MID0/TP 
                 Destination 
               
               
                 SC 
                   
                   
                   
                   
                   
                   
                 or Source 
               
               
                 messages 
                   
                   
                   
                   
                   
                   
                 Equipment Type 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Class 6 
                 C6MID2 
                 C6MID1 
                 C6MID0 
                 DD9 
                 DD8 
                 DD7 
                 DD6 
                 DD5 
               
               
                 Diagnostic 
               
               
                 messages 
               
               
                 Class 7 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Message 
                 CAN extended 29-bit message ID 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Class 
                 10 
                 9 
                 8 
                 7 
                 6 
                 5 
                 4 
                 3 
                 2 
                 1 
                 0 
               
               
                   
                   
               
               
                   
                 Class 0 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Class 1 
                 Destination 
                 UIID3 
                 UIID2 
                 UIID1 
                 UIID0 
                 SS1 
                 SS0 
                 DS1 
                 DS0 
               
               
                   
                 UI/G 
                 or Source 
               
               
                   
                 messages 
                 Equipment Type 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Class 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Class 3 
                 Source 
                 SS1 
                 SS0 
               
               
                   
                 Broadcast 
                 Equipment 
               
               
                   
                 messages 
                 Type 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Class 4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Class 5 
                 Destination 
                 SS1 
                 SS1 
                 DS1 
                 DS0 
               
               
                   
                 SC 
                 or Source 
               
               
                   
                 messages 
                 Equipment Type 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Class 6 
                 DD4 
                 DD3 
                 DD2 
                 DD1 
                 DD0 
                 UIID3 
                 UIID2 
                 UIID1 
                 UIID0 
                 S/DS1 
                 S/DS0 
               
               
                   
                 Diagnostic 
               
               
                   
                 messages 
               
               
                   
                 Class 7 
               
               
                   
                   
               
            
           
         
       
     
     In various embodiments, all message IDs on the RSBus  180  follow the described encoding, with one exception. For a transfer protocol flow control frame, the message ID may duplicate the message ID of the frame of the received transfer protocol First Frame. 
     The various message classes of Table II are now described. Message Class 1 includes User Interface and Gateway messages, e.g., those sent by the UI/G  250 . These messages serve to communicate with the user during normal system operation. They include messages sent from the user interface or the gateway, as well as some messages explicitly and implicitly addressed to them. 
       FIG. 9  illustrates an embodiment of the disclosure of the AF  720  ( FIG. 7 ) for Class 1 messages. The control bits in the AF  720  are encoded as follows: 
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 Class 1 Message Arbitration Field Breakdown 
               
            
           
           
               
               
               
            
               
                 Sub-Field 
                 Description 
                 Purpose 
               
               
                   
               
               
                 DSI0-DSI1 
                 Destination Subnet 
                 Indicate the subnet the 
               
               
                   
                 Identifier 
                 message is sent to 
               
               
                 SSI0-SSI1 
                 Source Subnet 
                 Indicate the subnet the 
               
               
                   
                 Identifier 
                 message originated in 
               
               
                 UIID0-UIID3 
                 User Interface ID 
                 Indicate the address of 
               
               
                   
                   
                 the UI or G the message 
               
               
                   
                   
                 is sent to or from, 
               
               
                   
                   
                 values 0-11 denote User 
               
               
                   
                   
                 Interfaces, values 12-15 
               
               
                   
                   
                 identify Gateways; it is 
               
               
                   
                   
                 equivalent to the UI/Gs 
               
               
                   
                   
                 Equipment Type numbers, 
               
               
                   
                   
                 offset by 70h - e.g. if 
               
               
                   
                   
                 the UI has the ET = 72h, 
               
               
                   
                   
                 its UIID = 2 
               
               
                 Equipment Type 
                 Equipment Type 
                 As defined in Table I 
               
               
                   
                 Number 
                   
               
               
                 C1MID0 = ID17/TP 
                 Class 1 Message 
                 Least significant bit of 
               
               
                   
                 ID LSb/Transfer 
                 the Class 1 Message ID. 
               
               
                   
                 Protocol 
                 This bit indicates if the 
               
               
                   
                   
                 message is a Transfer 
               
               
                   
                   
                 Protocol message (TP = 
               
               
                   
                   
                 1) or not (TP = 0) 
               
               
                 C1MID0-C1MID8 = 
                 Class 1 Message 
                 Unique 9-bit message 
               
               
                 ID17-ID25 
                 ID 
                 identifier within Class 1 
               
               
                   
               
            
           
         
       
     
       FIG. 10A  illustrates an embodiment of the disclosure of the AF  720  ( FIG. 7 ) for Class 3, System Broadcast messages. System Broadcast Messages are broadcasted from one subnet, such as the subnet  400 , but all local controllers  290  from all subnets can listen and respond to a Class 3 message. System Broadcast messages include DEVICE_Status and Alarms messages. There are 8,192 (2 13 ) System Broadcast messages possible in the illustrated embodiment. The number of alarms is limited to a subset of the total possible number of message, e.g. 1024)(2 10 ). The control bits in the AF  720  are encoded as follows for this message class: 
     
       
         
           
               
             
               
                 TABLE IV 
               
             
            
               
                   
               
               
                 Class 3 Message Arbitration Field Breakdown: 
               
               
                 System Broadcast Messages 
               
            
           
           
               
               
               
            
               
                 Sub-Field 
                 Description 
                 Purpose 
               
               
                   
               
               
                 SSI0-SSI1 
                 Source Subnet 
                 Indicate the subnet the 
               
               
                   
                 Identifier 
                 message originated from 
               
               
                 AL 
                 Alarms 
                 AL = 0 indicates that the 
               
               
                   
                   
                 message is a system 
               
               
                   
                   
                 broadcast message. 
               
               
                   
                   
                 AL = 1 indicates an alarm 
               
               
                 AS 
                 All subnets 
                 AS = 0 indicates that the 
               
               
                   
                   
                 message is broadcast on 
               
               
                   
                   
                 all subnets. 
               
               
                   
                   
                 AS = 1 indicates that the 
               
               
                   
                   
                 destination subnet is 
               
               
                   
                   
                 identical to the source 
               
               
                   
                   
                 subnet 
               
               
                 Equipment Type 
                 Equipment Type 
                 As defined in Table I 
               
               
                   
                 Number 
                   
               
               
                 C3MID0-C3MID12 = 
                 Class 3 Message 
                 Unique 13-bit message 
               
               
                 ID12-ID24 
                 ID 
                 identifier within Class 3 
               
               
                   
               
            
           
         
       
     
       FIG. 10B  illustrates an embodiment of Class 3 messages for the case that the message is an Alarm message. In various embodiments, all Alarm messages are Class 3 messages. An Alarm message includes additional information about the alarm priority encoded in the PR0-PR1 bits. The control fields in the AF  720  are encoded as indicated in Table V. 
     
       
         
           
               
             
               
                 TABLE V 
               
             
            
               
                   
               
               
                 Class 3 Message Arbitration Field Breakdown: Alarm Messages 
               
            
           
           
               
               
               
            
               
                 Sub-Field 
                 Description 
                 Purpose 
               
               
                   
               
               
                 SSI0-SSI1 
                 Source Subnet 
                 Indicate the subnet the 
               
               
                   
                 Identifier 
                 message originated from 
               
               
                 AL 
                 Alarms 
                 Set to 1 to indicate 
               
               
                   
                   
                 an alarm 
               
               
                 PR0-PR1 
                 Alarm Priority 
                 Encodes alarm priority, 
               
               
                   
                   
                 e.g., minor, moderate 
               
               
                   
                   
                 and critical 
               
               
                 SC 
                 Set/Clear 
                 Set to 0 when the alarm is 
               
               
                   
                   
                 set and set to 1 when the 
               
               
                   
                   
                 alarm is being cleared 
               
               
                 ID12-ID21 
                 Alarm Number 
                 The exact representation 
               
               
                   
                   
                 of the alarm number 
               
               
                 AS 
                 All Subnets 
                 AS = 0 indicates that the 
               
               
                   
                   
                 message is broadcast on all 
               
               
                   
                   
                 subnets. 
               
               
                   
                   
                 AS = 1 indicates that the 
               
               
                   
                   
                 destination subnet is 
               
               
                   
                   
                 identical to the source 
               
               
                   
                   
                 subnet 
               
               
                 Equipment Type 
                 Equipment Type 
                 As defined in Table I 
               
               
                   
                 Number 
               
               
                   
               
            
           
         
       
     
       FIG. 11  illustrates an embodiment of the disclosure of the AF  720  for Class 5, Subnet Controller messages. Messages in this class may be used primarily when a local controller  290  is in a COMMISSION state or a CONFIGURATION mode. In some embodiments, all messages in class 5 are used for communication to or from the Subnet Controller, e.g., the SC  230 . The format of messages in Class 5 may be constrained to be as defined in Table II. In  FIG. 10B , ID 14 -ID 25  identify a unique message resulting in total of 4096 (2 12 ) messages in this class. Table VI describes the bit assignments in the AF  720  for Class 5 messages. 
     The Equipment Type and the Destination Subnet Identifier denote the specific device and the specific HVAC system (network subnet) to which the message is addressed when sent from the SC  230 . If the message is sent to the SC  230 , the Equipment Type identifies the device sending the message and the SSI bits identify the subnet of the device. The SC  230  being addressed is identified by the Destination Subnet Identifier bits. 
     During normal operation, a Subnet Identifier in device messages would typically not change unless the particular local controller  290  to which the device messages pertain is reconfigured to work on a different subnet. The Equipment Type designator assigned to a local controller  290  typically does not change, but may be reassigned if the local controller  290  is reconfigured. Generally, local controllers  290  other than Subnet Controllers respond only to class 5 messages containing their Equipment Type and Subnet ID in the destination field. 
     
       
         
           
               
             
               
                 TABLE VI 
               
             
            
               
                   
               
               
                 Class 5 Message Arbitration Field 
               
               
                 Breakdown: Subnet Controller Messages 
               
            
           
           
               
               
               
            
               
                 Sub-Field 
                 Description 
                 Purpose 
               
               
                   
               
               
                 DSI0-DSI1 
                 Destination Subnet 
                 Indicate the subnet the 
               
               
                   
                 Identifier 
                 message is sent to 
               
               
                 SSI0-SSI1 
                 Source Subnet 
                 indicate the subnet the 
               
               
                   
                 Identifier 
                 message originated in 
               
               
                 Equipment Type 
                 Equipment Type 
                 As defined in Table I 
               
               
                   
                 Number 
                   
               
               
                 C1MID0 = ID13/TP 
                 Class 5 Message 
                 Least significant bit of 
               
               
                   
                 ID LSb/Transfer 
                 the Class 5 Message ID. 
               
               
                   
                 Protocol 
                 If 1, indicates the 
               
               
                   
                   
                 message is a Transfer 
               
               
                   
                   
                 Protocol message 
               
               
                 C5MID0-C5MID12 = 
                 Class 5 Message 
                 Unique 13-bit message 
               
               
                 ID13-ID25 
                 ID 
                 identifier within Class 5 
               
               
                   
               
            
           
         
       
     
     Diagnostic messages are categorized as Class 6 messages. Class 6 messages use Device Designator bits to identify the destination device. Even when the local controller  290  is not configured, or is disabled as described below, the local controller  290  can still send or receive Class 6 messages. In various embodiments, the local controller  290  can send and receive Class 6 messages before being configured or while disabled. The control bits in the AF  720  are encoded as described in Table II, and further detailed in Table VII. 
     
       
         
           
               
             
               
                 TABLE VII 
               
             
            
               
                   
               
               
                 Class 6 Basic Diagnostic Messages 
               
            
           
           
               
               
               
            
               
                 Sub-Field 
                 Description 
                 Purpose 
               
               
                   
               
               
                 UIID0-UIID3 
                 User Interface ID 
                 Indicate the address of the 
               
               
                   
                   
                 UI/G the message is sent to 
               
               
                   
                   
                 or from. Values 0-11 may 
               
               
                   
                   
                 denote User Interfaces; 
               
               
                   
                   
                 values 12-15 may identify 
               
               
                   
                   
                 Gateways. The User 
               
               
                   
                   
                 Interface ID is equivalent to 
               
               
                   
                   
                 the UI/G Equipment Type 
               
               
                   
                   
                 numbers, offset by 70h. 
               
               
                   
                   
                 E.g., if the Equipment Type 
               
               
                   
                   
                 of the UI is 72h, its UIID 
               
               
                   
                   
                 is 2 
               
               
                 DD0-DD9 
                 Device Designator 
                 Indicate the device&#39;s 10 
               
               
                   
                   
                 least significant Device 
               
               
                   
                   
                 Designator bits 
               
               
                 S/DSI0-S/DSI1 
                 Source/Destination 
                 Indicate the subnet of the 
               
               
                   
                 Subnet Identifier 
                 UI/G that diagnoses the 
               
               
                   
                   
                 device 
               
               
                 C6MID0-C6MID9 = 
                 Class 5 Message ID 
                 reserved for a total of 1024 
               
               
                 ID16-ID25 
                 LSb/Transfer 
                 possible message IDs in 
               
               
                   
                 Protocol 
                 this class 
               
               
                   
               
            
           
         
       
     
     Some messages sent over the RSBus  180  may expect a response. In some cases, the sender expects the response immediately, meaning as soon as the hardware and communication protocol allow the transmission of the response. Such messages are referred to herein as queries. Queries generally have various timing constraints associated therewith. One embodiment of a set of rules is described below that may apply to query messages for most purposes. 
     Message Response Time: Generally messages are to be sent without delay by the sending local controller  290 . In many cases it is preferable if a response to a query is generated in 100 ms or less. This means that upon receipt of a query, the responding local controller  290  should within 100 ms place a response into its CAN transmit buffer. In many cases the response will not be sent within 100 ms, as the response timing is generally dependent on the traffic conditions on the bus and the message&#39;s priority. 
     In one exception to this general rule, in the SUBNET_STARTUP state the local controller  290  is generally configured to wait 100 ms before it attempts to respond to the Coordinator message. Thus, the response will be placed in the transmit buffer at a time greater than 100 ms after receipt of the query. Other exceptions to the general response timing rule may be made as desired. 
     Message Resend: A local controller  290  may be configured to resend a message when a correct reply to the message is not received within the expected timeout period. The timeout period may be set to any non-zero value, e.g., about 1 second. If the message is resent after an initial message, and no response is received within the timeout period after the subsequent message, the local controller  290  may attempt to resend the message again. If a response to the third attempt is not received within the timeout period, the local controller  290  may be configured to cease further resending of the message. Of course, more or fewer attempts may be made before ceasing to send the message. In some embodiments, the local controller  290  may send an alarm message identifying the Equipment Type of the unresponsive device, or act in any other way desired. 
     Subnet Controller Monitoring: In some embodiments, the aSC  230   a  sends a periodic message to other devices on the RSBus  180  that indicates the aSC  230   a  is present and functioning normally. This message is referred to for convenience as a “Heartbeat” message, e.g., aSC_Heartbeat. Each enabled local controller  290  may listen to the aSC_Heartbeat message and, when the message is not detected for a specified listening period, may take a specified action. In one embodiment, the local controller  290  may issue an alarm when the Heartbeat message is absent for more than three times its usual send period, e.g., three messages are missed. In some embodiments, the local controller  290  also ceases operation and returns to a default state. 
     Timing Accuracy: Each local controller  290  typically includes an oscillator to provide a timing reference. Each oscillator is preferred to conform to the accuracy for systems with bus speed of up to 125 kbaud, as defined in section 9.1 of the Bosch CAN 2.0B specification. This specification defines resonator accuracy over the entire temperature range and all environmental conditions, including aging, to be 1.58%. In some embodiments, a maximum additional ±200 μs tolerance may be accommodated without notable system degradation. In some cases, the tolerance of the device oscillator may be made more stringent when real-time clock functions are provided. 
     RSBus IDs Header File: The local controllers  290  may be provided by numerous manufacturing suppliers. To promote uniformity of configuration of the various local controllers  290 , a system integrator may provide a uniform header file that the suppliers include in firmware controlling the operation of the local controller  290 . It is generally preferable that the suppliers use the uniform header file without modification in furtherance of the objective of uniformity of the integrated devices. In an embodiment, the uniform header file contains all RSBus Message IDs for all messages, including the class of each message. In an embodiment, the file also contains the most current parameter and feature numbers as well as the system wide alarms. In some cases, the alarms, features, parameters and messages are identified by their string names in all caps format, with a prefix according to the type. Thus, e.g., alarm names may be prefixed with a lower-case letter “a”, feature names may be prefixed with “f”, parameter names with “p” and message names with “mIDx_” when defining the message ID from a class x and “mc” when defining the message class. 
     In a specific example for illustration purposes only, a class 3 message FOO with a message ID of 0x100 may be defined as follows: 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                   
                 #define 
                 mcDEVICE_FOO 
                 3 
               
               
                   
                 #define 
                 mID3_DEVICE_FOO 
                 0x100 
               
               
                   
               
            
           
         
       
     
     The file may include the following sections:
         Own Alarm IDs—includes Alarm IDs for all Alarms generated by the device   Parameter IDs—includes Parameter IDs for all parameters sent (owned) and received by the device   Feature IDs—includes Feature IDs for all parameters sent (owned) and received by the device   Own User Text IDs—includes all User Text IDs stored by the device   Sent/Received Message IDs—includes message IDs for all messages sent and received by the device   Sent/Received Message Classes—includes message classes for all messages sent and received by the device   Own Alarm Texts—includes installer text in all device supported languages for all alarms owned by the device   Own Feature Texts—includes installer text in all device supported languages for all features owned by the device   Own Parameter Texts—includes installer text in all device supported languages for all parameters owned by the device   Own Feature Send/Receive Matrix—defines whether the particular Feature is sent and/or received by the device   Own Parameter Send/Receive Matrix—defines whether the particular Parameter is sent and/or received by the device   Own Message Send/Receive Matrix—defines what states each message is sent and/or received by the device       

     Message Bit Timing: Bit timing of the local controllers  290  may be specified for uniformity of operation. In an embodiment, the local controllers  290  configure the CAN bit timing as follows. The local controller  290  timing oscillator produces a periodic signal with a period referred to as a time quantum (TQ). In some embodiments, one bit has a period of 25 TQ. Thus for a timing oscillator having a period, or TQ, of about 1 μs, the bit rate is about 40 kBaud. A local controller  290  may sample the data on the RSBus at a time related to the TQ. In an embodiment, there the local controller  290  uses a delay time of 8 TQ. The bit is sampled between the 17th and 18th TQ of the bit. If multiple sample points are selected, they may be centered on the transition from 17th to 18th TQ. 
     If the chosen CAN platform does not support the clock divider that allows 25 TQ per bit timing, it may be preferred to use a setting with the highest number of TQs per bit, preferably not greater than 25. Delay time should be adjusted to 32% of the bit duration, and the sample points at or centered on 68% of the bit duration. 
     Any of the local controllers  290  in the network  200  may be reset by cycling the power thereof. The local controller  290  is typically configured to execute a power-up routine that places the local controller  290  in a state ready to be configured via messages and thereby begin normal operation. 
     However, among the many advantages of the communication protocol of the disclosure is the ability to implement an efficient method of resetting the local controllers  290  on the RSBus  180  without cycling power to the devices. In an embodiment, a software reset may be issued upon a timeout when a local controller  290  enters the bus off state as described earlier. 
     In another embodiment, the SC  230  resets a local controller  290  using a combination of two messages. A first reset message commands the local controller  290  to prepare for a reset. Optionally, the local controller  290  may respond to the first message with a message, e.g., DEVICE_Waiting_for_Reset, indicating that the local controller  290  is waiting for a reset message. The SC  230  sends a second reset message instructing the local controller  290  to reset. The local controller  290  may be configured to only reset in response to the second message if the second message is received within a predetermined time period after the first message, e.g., one minute. If the second message is not received within the predetermined time period, the local controller  290  may resume normal operation and ignore any reset messages received before another message to prepare to reset. In some cases the first and second messages may respectively instruct multiple local controllers  290  to prepare for reset, and to reset. 
     In another embodiment, the local controller  290  is configured to be placed in a “HARD_DISABLED” and a “SOFT_DISABLED” state. The HARD_DISABLED state may be initiated by a user via a message from the UI/G  250 . In some cases, this aspect provides the ability to enable or disable a local controller  290  without physically locating the local controller  290 . This may be particularly advantageous when the local controller  290  is a logical device, the location of which may be difficult to immediately determine, when expedient disabling of the local controller  290  is desired. 
     While in the HARD_DISABLED state, the local controller  290  may be configured to monitor the RSBus  180  without transmitting messages thereover. However, while in this state the local controller  290  may not send messages to other device, nor may the local controller  290  perform any control of an associated demand unit  155 . The UI/G  250  may send an appropriately configured message to the local controller  290  instructing the local controller  290  to enter or exit the HARD_DISABLED state. These messages may be, e.g., class 6 messages. When the local controller  290  receives a message altering its HARD_DISABLED state, it may respond by issuing an acknowledgment over the RSBus  180 , e.g., via a DEVICE_UI/G_Enable_Acknowledge message. Subnet controllers  230  may monitor these messages to track the state of enablement of the various local controllers  290  on the RSBus  180 . When the local controller  290  is hard enabled, it may reset itself and enter a CRC check mode. 
     The HARD_DISABLED state is persistent, meaning that the local controller  290  remains in the HARD_DISABLED state until the user takes an action to send an enable message. In various embodiments the state of enablement is logged in the NVM  320  associated with each physical or logical local controller  290 . Thus, the state of enablement, including the condition of being disabled, is remembered by the local controller  290  after reset. 
     In the SOFT_DISABLED state, the local controller  290  may continue to respond to messages from the SC  230 , but may not execute any control functions. The message may be of a reserved type that instructs one or more local controllers  290  to proceed to a startup state. The local controller  290  may respond in such cases by issuing a message alerting the SC  230  that the local controller  290  is starting up. In various embodiments the state of enablement in the SOFT_RESET state is stored in the RAM  320 . Thus, the state of disablement in the SOFT_RESET state may be cleared upon reset of the system  100 . 
     The SOFT_DISABLED state may be cleared when the local controller  290  is reset. The aSC  230   a  may implement a soft-disable of a local controller  290  when the local controller  290  is “alien” to the subnet, e.g., unrecognized as a properly initiated local controller  290 . The aSC  230   a  may also soft-disable a local controller  290  that is determined to be malfunctioning. 
     In some embodiments, entry of the local controller  290  to a privileged operating mode may be controlled by messages issued by a special-purpose command interface. One such mode is an OEM programming mode. The OEM programming mode may be used to download configuration data to the local controller  290 . Configuration data may include, e.g., serial and model numbers, unit capacity, etc. Such information may be stored in non-volatile memory of the local controller  290 , e.g., the NVM  320 . 
     Another privileged operating mode is an OEM functional test mode. This mode may provide the ability to test the local controller  290  using, in addition to the messaging protocol, a special data sequence input to a test port that may be separate from the communication capability of the controller  290 . For example, the command interface may send a demand message to and receiving status information from the local controller  290  over the RSBus  180 , as discussed more fully below. 
     In some embodiments, a special command sequence from a standard UI/G  250  may be used to implement either privileged operation mode. Use of these modes may be restricted by password protection if desired. 
     When the HVAC system  100  is reset or powered up, the local controllers  290  on the subnet are configured in various embodiments to establish an initial operating state of the system  100 . One aspect includes configuration of one or more SCs  230  of the subnet of the network  200 , e.g. As described further below, each SC  230  enters a SUBNET_STARTUP state upon power-up. During the SUBNET_STARTUP state, the one or more SCs  230  negotiate for the control of the subnet. This negotiation is based on a set of features and parameters of each SC  230 , and is designed to ensure that the best SC  230  available controls the subnet. After this negotiation is completed, the SC  230  that is selected by the negotiation process becomes active, or in other words, becomes an aSC that thereafter takes firm control of the subnet. At that point the SC  230  places the subnet in a CONFIGURATION mode or a VERIFICATION mode, and proceeds to assign or reassign Equipment Types and Subnet IDs to the local controllers  290  on the subnet. 
     In the CONFIGURATION mode, a SUBNET_STARTUP process serves to configure the subnet to an operational state. In the VERIFICATION mode, the SUBNET_STARTUP process verifies that a current subnet configuration matches a subnet configuration set up previously during an initial configuration. It is possible to add new devices to the subnet during a Configuration routine executed only when in the CONFIGURATION mode. The VERIFICATION mode may be similar to the CONFIGURATION mode, with two differences as follows. First, in some embodiments the aSC  230   a  reassigns the same Equipment Types and Subnet ID numbers to the local controllers  290  as were assigned thereto during the last initial configuration. Second, the VERIFICATION mode may be configured to exclude the registration of new devices on the network. As described further below, the CONFIGURATION mode or the VERIFICATION mode may be indicated by values of a CF0 and a CF1 flag, defined below, of the one or more SCs  230  present in the subnet. 
       FIG. 12  illustrates a state diagram of an embodiment of a SUBNET_STARTUP process, generally designated  1200 , that is configured to run on a local controller  290 . In various embodiments the process  1200  is implemented as a finite state machine (FSM). In some embodiments the FSM is only implemented on local controllers  290  that are not a subnet controller during the process  1200 . In some embodiments, every local controller  290  that is not an SC  230  runs a FSM machine consistent with the process  1200 . The process  1200  may execute in response to messages sent by the SC  230 . 
     The process  1200  begins with a reset state  1210 . As mentioned previously, the reset state may be reached from a power-up condition, another device state (such as a check of the NVM  320 ) or a reset command from a controller, e.g. the aSC  230   a . The process  1200  advances to a state  1220 , designated DEVICE_PRE_STARTUP. In an illustrative embodiment, the state  1220  includes a plurality of configuration events that ends with the local controller  290  sending a message over the RSBus  180  indicating the local controller  290  is ready to start. This message is referred to for convenience as DEVICE_Startup. After the state  1220 , the process  1200  advances to a state  1230 , designated WAIT_TO_BE_ASSIGNED. The state  1230  includes a plurality of configuration events that ends with the local controller  290  receiving a message from the aSC  230   a  commanding a change to an operational state, referred for convenience as aSC_Change_State. Upon receiving the aSC_Change_State message, the process  1200  advances to a state  1240  and exits. 
     Note that in all states the local controller  290  can still respond to a Class 6 diagnostic message. Thus, from any state, a message may force the process  1200  to the HARD_DISABLED state  1250  or to the reset state  1210 . The process  1200  illustrates an example in which the process  1200  enters the HARD_DISABLED state  1250  from the state  1220 . The process  1200  remains in the state  1250  until the local controller  290  receives an appropriate message as described previously. The process  1200  may then advance to the state  1210 , from which the local controller  290  may begin the initialization process again. 
     The process  1200  also illustrates an example in which the process  1200  enters a state  1260  SOFT_DISABLED state from the state  1230 . The process  1200  may remain in the state  1260  until the local controller  290  is reset as previously described. The process  1200  may then advance to the state  1220 . 
     In some embodiments, the local controller  290  is configured to remain, during the state  1220 , in a listen-only mode for a predetermined period, e.g., at least about 5000 ms. In the listen-only mode, the local controller  290  monitors messages sent over the RSBus  180 , but does not initiate any messages. After the listen-only period expires, the local controller  290  may optionally wait an additional startup delay period. After the optional additional delay, the local controller  290  may send a DEVICE_Startup message over the RSBus  180 , and may then monitor the RSBus  180  for any messages that indicate other devices on the RSBus  180  failed to receive the startup message correctly. 
     In some cases, the local controller  290  may initiate its startup message before the end of the 5000 ms listen-only period. In one embodiment, the local controller  290  receives an SC_Coordinator message. In this case, the local controller  290  sends a startup message immediately after powering up. In this context, immediately means after about 100 ms plus an additional delay derived from the Device Designator. In some such cases, the message is not received successfully by at least one other local controller  290 , resulting in a Bit Error event on the RSBus  180 . If the Bit Error is detected, then the device may wait a specified period, after which it resends the startup message. A specific resend delay period may be selected for a particular local controller  290 . In various embodiments the resend delay reduces the probability of message collision on the data bus  180 . An algorithm that determines this resend delay time as a function of the Device Designator may compute the resend delay period as described further below. 
     In various embodiments the system  100  is configured to allow multiple devices of the same type to start communicating on the network  200 . These embodiments allow seamless plug-and-play configuration even when the bandwidth of the data bus  180  is limited. The following example illustrates principles of these embodiments, and is presented by way of illustration without limitation. 
     In one embodiment the Device_Startup message ID is unique to each type of device. The message data field of the Device_Startup message may be identical to the data field of the Device_Designator message. These messages may be Class 5 messages, and in such cases they may have RSBus message IDs that include an offset number and a five bit order number shifted left by one, so as not to interfere with the CAN ID bit ID 13  used to indicate the transport protocol. In these messages the order number is defined for a particular system device  410  as the five least significant bits of the Device Designator (DD) of that system device  410 . In some embodiments using a Class 3 message that includes the 5-bit order number, the position of the order number in the message ID is not shifted by one. For the case of the aSC  230   a , the order number can also be a number calculated from the number of other subnet controllers, typically one or more instances of the iSC  230   i , detected on the subnet. 
     In a nonlimiting example, the Device_Startup message is 0x180, and the last byte of the DD is 0x45, then the message ID of the Device_Startup message is 0x180+(0x45 &amp; 0x1F)=0x180+0x0A=0x18A. 
     As described further below, a base delay time may be scaled by the value of the order number. The occurrence of a bit error when a system device  410  sends a Device_Startup message indicates that two devices  290  simultaneously attempted to publish a message on the data bus  180 , referred to herein as a message collision, or more briefly, a collision. When a collision occurs, the devices delay resending the Device_Startup message for a unique period derived from the order number. 
     Thus in various embodiments the presence of the order number advantageously reduces the probability of collisions by the factor 2 b , where b is the number of bits of the order number. In the embodiments described above, the collision probability is reduced to about 3% of the collision probability that would otherwise be present. 
     After the local controller  290  successfully sends its startup message, the local controller  290  may wait for a Startup Response message from the aSC  230   a . The Startup Response may be configured to provide a node assignment to the local controller  290 . A node assignment message is referred to for discussion purposes as an aSC_DEVICE_Assignment message. The aSC_DEVICE_Assignment message may be sent by the aSC  230   a  from its subnet. This message may contain information regarding the subnet that the local controller  290  may need to operate properly. Information conveyed by aSC DEVICE Assignment may additionally include the Equipment Type assigned to local controller  290 , and other flags. After the local controller  290  receives the aSC_DEVICE message, the local controller  290  may send an acknowledgement message over the subnet of the network  200  it has been assigned to. The message may include, e.g., the Equipment Type of the local controller  290 . 
     If a local controller  290  does not detect a startup response message addressed to it within 5 minutes after it is initiated, the local controller  290  may repeat its startup message. In some embodiments, the local controller  290  repeats the startup message every 5 minutes until the local controller  290  successfully receives an Equipment Type and Subnet ID assignment, e.g., via a aSC_DEVICE_Assignment message. The local controller  290  may send an acknowledgement message indicating it is configured and ready to operate normally. In some embodiments, all local controllers  290  are required to receive an aSC_DEVICE_Assignment message before sending an acknowledgement. In some cases, exceptions may be made to this requirement where system design considerations warrant. 
     In some cases, the local controller  290  was assigned an Equipment Type and a Subnet ID in a previous system startup. In some embodiments, the local controller  290  retains the previously assigned values. In other cases, the local controller  290  was not previously assigned an Equipment Type and a Subnet ID, such as when the local controller  290  is initially added to the system  100 . In some embodiments, a local controller  290  that has not previously been assigned an Equipment Type and a Subnet ID are assigned a default value. The default value of the Equipment Type may be a lowest Equipment Type for the specific device. The default Subnet ID may be, e.g., 0. 
     When a subnet starts up, an SC  230  may publish an SC_Coordinator message to the data bus  180  to coordinate control of the subnet with any other instances of the SC  230  on the subnet. When the local controller  290  receives the SC_Coordinator message it may respond with a DEVICE_Startup message if the local controller  290  is in the SUBNET_STARTUP or SOFT_DISABLED states, or in an OEM Test state described above. Otherwise the local controller  290  may respond with the DEVICE_Device_Designator message. 
     For example, if the local controller  290  sees the SC_Coordinator message after powering up, it may respond with the DEVICE_Startup message. Then, the local controller  290  may be assigned an Equipment Type and Subnet ID by an aSC_DEVICE_Assignment message, and may then receive an aSC_Heartbeat message. If the local controller  290  receives another SC_Coordinator message, the local controller  290  may again respond with the DEVICE_Startup message, because it has not cleared the SUBNET_STARTUP state since the last reset. If the local controller  290  is assigned and changes state to, e.g. a COMMISSIONING state and then receives another SC_Coordinator message, the local controller  290  may respond with a DEVICE_Device_Designator message. If the local controller  290  is assigned for the second time, but remains in the SUBNET_STARTUP state when yet another SC_Coordinator message arrives, it may respond with a DEVICE_Startup message, as it is no longer necessary to remember the previous state. 
     Restated from the perspective of the aSC  230   a , if the aSC  230   a  receives a DEVICE_Device_Designator message, it knows that the local controller  290  has not recently been reset. If the aSC  230   a  receives the DEVICE_Startup message, it knows that the local controller  290  has not been assigned and has not changed state since last hardware or software reset of the local controller  290 . 
       FIG. 6B  illustrates an embodiment in which a link relay is used to selectively isolate the subnet  615  from the subnet  620 . In a conventional communicating HVAC system all devices share a common communicating bus. During system installation and configuration special care must be taken to ensure that corresponding equipment from same HVAC system is matched. Installation becomes more cumbersome and prone to error as the number of connected systems increases, and as the number of components in the total system increase. Moreover, if a bus error occurs, such as a short circuit between bus wires, the entire network may be disabled. 
     In the embodiment of  FIG. 6B , the subnet  615  and the subnet  620  may be selectively isolated from each other using a switch  699  such as a relay. Two systems, one corresponding to each subnet  615 ,  620  may be installed, configured and tested separately. At a proper time the aSC  230   a  in each subnet  615 ,  620 , for example the SC  662  and the SC  682 , may link its subnet to another subnet by actuating the switch  699 . Advantageously, and in contrast to conventional HVAC systems, if a communication bus failure is detected, the aSC  230   a  may disconnect its subnet from the network  200  to localize the problem. The aSC  230   a  may put the switch  699  in a local mode (e.g., isolating its subnet) as soon as immediately upon receiving a SC_Coordinator message from any subnet, or upon receiving an SC_Startup message from an aSC  230   a  on the same subnet. After repair, the aSC  230   a  may be instructed via an appropriately configured message to reconnect to the network  200 . Thus, at least some HVAC services may be maintained even if one subnet is rendered inoperable by a failure of the data bus  180 . 
     Turning now to  FIGS. 13A and 13B , illustrated is a method generally designated  1300 A that may run on a subnet controller, e.g., the SC  230 , during subnet startup, e.g., during the SUBNET_STARTUP state.  FIG. 13A , presenting a summary view of the method  1300 A, is described first.  FIG. 13B , described afterward, presents a more detailed flow chart  1300 B of the method. 
     First addressing  FIG. 13A , the method  1300 A begins with a reset state  1301 . The state  1301  may result from power-up or an appropriately configured reset command. A state  1303  provides pre-startup activity, e.g., startup messages to system devices  410  in the network  200 . A state  1309  provides post-startup activity, e.g., arbitrating the aSC  230   a . In some cases a system device  410  will be placed in a hard disable state  1307 , for example when pre-startup activity indicates that the system device  410  is not functioning properly. After the post-startup state  1309 , an SC  230  that is assigned the role of the aSC  230   a  during arbitration during the state  1309  proceeds to an active-coordinator state  1313 . The aSC  230   a  may perform system administrative tasks in the state  1313 . After the aSC  230   a  performs such administrative tasks the method  1300 A proceeds to state  1379  at which point the aSC  230   a  broadcasts an aSC_heartbeat message, indicating that the aSC  230   a  has asserted control over its subnet. The method  1300 A terminates with a state  1399 , from which the aSC  230   a  continues with system control functions. 
     An SC  230  that does not become the aSC  230   a  advances in the method  1300 A to a passive-coordinator state  1315 . The SC  230  entering the state  1315  is assigned the role of iSC  230   i . The iSC  230   i  performs various tasks in the state  1315  and may then advance to an inactive state  1355 . In various embodiments the iSC  230   i  continues to receive messages in the inactive state  1355 , and may perform some functions such as storing backup parameters from other system devices  410 , but does not exert control over the subnet. In some cases the iSC  230   i  may advance to a soft disable state  1351 , e.g. if commanded to do so by a suitable formatted message. 
     As described,  FIG. 13B  presents a more detailed flow chart of the method  1300 A, generally designated in  FIG. 13B  as a method  1300 B. During the pre-startup state  1303 , the SC  230  may execute a step  1305 , in which it may send several SC_startup messages according to the various embodiments described herein. In the post-startup state  1309 , the SC  230  may perform the previously describe arbitration in a step  1311 . If the SC  230  becomes the aSC  230   a , it may in various embodiments be the first of multiple instances of the SC  230  on the subnet to broadcast an SC_coordinator message. In a step  1317 , the SC  230  determines if it is indeed the first to broadcast the SC_coordinator message. If so, the SC  230  enters the active-coordinator state  1313 , wherein it performs various administrative tasks  1357 - 1375 . The SC  230  then advances to the heartbeat-out state, wherein it may broadcast the aSC_heartbeat message in a step  1377 . The SC  230 , now referred to as the aSC  230   a , may perform various configuration steps  1381 - 1391  before exiting the method  1300 B with an exit state  1399 . 
     If in the step  1317  the SC  230  determines it is not the first SC  230  to send an SC_coordinator message, it branches to the passive-coordinator state  1315  described previously. The SC  230  may perform various configuration steps  1319 - 1347 . In a step  1349 , the SC  230  may determine that it is disabled. If so, the SC  230  may enter the soft-disabled state  1351  and remain therein until a reset. If the SC  230  is not disabled, it may enter the inactive state  1355 , at which point it is referred to as the iSC  230   i . The method  1300 B exits with an exit state  1398 . 
       FIG. 13C  presents without limitation an example embodiment of states of a state machine configured to implement a subnet controller startup process. Those skilled in the pertinent art will appreciate that the illustrated embodiment is one of many that may be used, and that such others are included in the scope of the disclosure. 
     In an advantageous embodiment, the controllers SC  230  do not queue inbound or outbound messages. Configuration times discussed below are presented without limitation for this case. Moreover, if a message is scheduled to be sent out at a specified time, in some embodiments only one attempt to send the message is made. The SC  230  does not automatically attempt to resend the message in such embodiments. However, the SC  230  may attempt to resend the message when a new specific time is scheduled to send the message after the send failure. 
     In an embodiment, the Subnet Controller startup sequence begins with the SC  230  issuing a SC_Startup message. The message may be sent at a consistent period after the SC  230  emerges from a reset state. In an example embodiment, the period is about 3000 ms plus a supplemental delay period derived from the Device Designator. 
     After performing a functional test of local NVM, e.g. the NVM  320 , each SC  230  on the RSBus  180  listens for startup messages from other local controllers  290 . The SC  230  records all Device Designators and configurations, e.g. Equipment Types and Subnet IDs, for all local controllers  290  on the network that send their startup messages. 
     After the supplemental delay period, e.g., about 1000 ms, the first SC  230  may attempt to send a second message, e.g., a SC_Coordinator message. In an example case in which there is no other traffic on the RSBus  180 , the SC_Coordinator message appears on the RSBus  180  after about 1000 ms plus the time required to send the SC_Startup message onto the RSBus  180 . Of course such timing is subject to imprecision determined by system-level design consideration. If the first SC  230  successfully broadcasts the SC_Startup message, it becomes the active coordinator, e.g., the aSC  230   a , and proceeds to coordinate the system configuration. If the first SC  230  fails to send the SC_Startup message, or a second SC  230  successfully sends a message first, then the second SC  230  becomes the aSC  230   a  and the first SC  230  enters a PASSIVE_COORDINATOR state and becomes an inactive subnet controller, e.g. the iSC  230   i.    
     The SC  230  may determine that it is a best subnet coordinator, e.g., has priority over other available instances of the SC  230  on the subnet  200 , by querying such other instances to determine relative capability and features. The SC  230  may additionally take into account factors unrelated to features and capability. Such determination may include the following factors, presented by way of example without limitation: 
     1) Subnet Priority Level (SPL) (akin to a user selectable override)—the operator can chose to use a particular SC  230 , even if it is deemed less advanced than others on the subnet 
     2) Device Product Level (DPL) (such as different tiers of capability based on cost)—an SC  230  with greater features or capability may be indicated by a product level number, with a greater number indicating a more capable SC  230   
     3) Its Protocol Revision Number (PRN)—a recent design revision of the SC  230  may be indicated by a higher revision number 
     4) Its Device Designator or Serial Number (DD/SN)—a greater number of the Device Designator or serial number may be generally associated with a more recently produced SC  230 , which may be presumed to be more capable 
     In some embodiments, the determination is made considering the above-listed factors in the order indicated. Thus, a first SC  230  with a greater DPL than a second SC  230  may take priority even if the second SC  230  have a greater PRN or DD/SN. In some embodiments, the SPL overrides all other factors. In some embodiments if all factors are otherwise equal, then the SC  230  with a greater Device Designator will take priority over any SC  230  with a lower Device Designator. 
     If the SC  230  determines that it is the most qualified SC  230  on the subnet  200 , it proceeds to assume control of the subnet  200  by first issuing a SC_Ready_To_Take_Over message. After a predetermined period, e.g. about 1000 ms, the aSC  230   a  issues the aSC_Heartbeat message. Alternatively, if the aSC  230   a  determines it is not the most qualified SC  230  on the subnet  200 , it will pass a token to the SC  230  that is determined to be the most qualified SC  230 . The SC  230  passing the token becomes an inactive iSC  230   i , and the SC  230  receiving the token becomes the aSC  230   a.    
     When the aSC  230   a  assumes control of the subnet of the network  200 , it determines if the subnet of the network  200  is in the CONFIGURATION mode or in the VERIFICATION mode and proceeds to configure the system accordingly. If the subnet of the network  200  is in the VERIFICATION mode, the aSC  230   a  issues alarms for all missing and new local controllers  290 . New local controllers  290  will be excluded from the subnet of the network  200  and placed in the SOFT_DISABLED state. The aSC  230   a  may also check the validity of the configuration of the subnet of the network  200  and issue appropriate alarms if needed. If the subnet of the network  200  is configured correctly, the aSC  230   a  concludes the SUBNET_STARTUP process by issuing an aSC_Change_State message. 
     In some cases there may be more than one SC  230  on a single subnet of the network  200  capable of controlling the subnet. In this case, an arbitration algorithm may arbitrate among the eligible SCs  230  to determine which SC  230  will assume the role of the aSC  230   a . The algorithm may consider various factors, including, e.g., for each eligible SC  230  the CF1 flag setting, defined below, a Subnet Priority Level, a Device Product Level, and a hardware revision number. A Subnet Priority Level may be, e.g., an identifier that allows for overwriting the priority level of an SC  230 . In some embodiments the Subnet Priority Level of each SC  230  is set to 0 in the factory and can only be changed by a specific sequence of messages sent by the Interface/Gateway  250 . The Device Product Level may be, e.g., a designation of a level of feature configuration, such as Signature, Elite or Merit product lines. After the system  100  is configured, all aSCs  230   a  run the normal operation of their respective network subnets. 
     In various embodiments each SC  230  in the system  100  stores the Device Designators of all other configured SCs  230  in the system  100 . Each SC  230  may also store its last active, inactive or disabled state. 
     Recalling that each message includes a message ID, the ID of the DEVICE_Startup message is unique to the message being sent. The message data field of the local controller  290  may be identical to the data field of DEVICE_Device_Designator messages sent by that local controller  290 . Since these messages may be Class 3 messages, as described previously, they may have RSBus Message IDs that are formed from an offset number and a 5-bit Order Number. In an example embodiment, the Order Number of a particular local controller  290  is the 5 least significant bits of the Device Designator of that local controller  290 . 
     For example, if the DEVICE_Startup message is 0x700 and the last byte of Device Designator is 0x45 then the message ID of the DEVICE_Startup message may be 0x700+(0x45 &amp; 0x1F)=0x700+0x05=0x705. 
     For Class 5 messages that include the 5-bit Order Number, the position of the Order Number in the message ID may be shifted left by one position so as to prevent interference with the position of the Transport Protocol bit C5MID0/TP. The Order Number of the SC  230  may also be a number calculated from the number of other SCs detected on the subnet. For details, see the device message document. 
     All startup messages, e.g., DEVICE_Device_Designator and SC_Coordinator messages, can contain seven Configuration Flags, CF0-CF6. The encoding of these flags may vary depending on the device type. For example, the flags of the SCs  230  may be encoded differently than other local controllers  290 . 
     In some embodiments, for the local controllers  290  that are not an SC  230  the flags may be encoded as follows:
         CF0: 0 if the local controller  290  has not been configured (e.g. is a new device)
           1 if Installer Test Mode tests complete successfully, or upon receipt of an aSC_Change_State message indicating transition to Normal Operation   
           CF1: 0 if the control is intended for permanent use
           1 if it is attached temporarily   
           CF2: 0 if the local controller  290  cannot be flashed over the RSBus  180 
           1 otherwise   
           CF3: 0 if the local controller  290  is hard disabled/not-communicating
           1 if the device is hard enabled/communicating   
           CF4: 0 if the local controller  290  is soft disabled, or was soft disabled immediately prior to sending this message, when this message is sent in the Subnet Startup state
           1 otherwise   
           CF5 0 if the local controller  290  is a factory installed part
           1 if the local controller  290  is a replacement part   
           CF6: 0 if the local controller  290  has failed the Data CRC check
           1 otherwise   
               

     In some embodiments, for the local controllers  290  that are an SC  230  the flags may be encoded as follows:
         CF0: 0 if the SC  230  has not been configured, e.g. is a new device
           1 upon successful completion of Installer Test Mode tests   
           CF1 0 if the SC  230  does not recognize any indoor units on the subnet
           1 if the SC  230  recognizes at least one indoor unit on the subnet   
           CF2 0 if the SC  230  cannot be flashed over RSBus  180 ,
           1 otherwise   
           CF3 0 if the SC  230  is hard disabled/not-communicating
           1 if the SC  230  is hard enabled/communicating,   
           CF4 0 if the Subnet Controller is soft disabled, or was soft disabled immediately prior to sending this message
           1 otherwise   
               

     As described above, the CF0 flag may be used as an indication of the whether an associated local controller  290  has been configured. The CF0 flag may be cleared (0) in all local controllers  290  under the following circumstances, e.g.:
         When all device parameters revert to default values, such as via a specific diagnostic inquiry/command.   When the device is restored to factory defaults via a specific diagnostic inquiry/command.   When the device loses its internal NVM settings, as described below.       

     At any time and regardless of the CF0 flag setting, if the local controller  290  enters the COMMISSIONING state and either the UI/G  250  or the aSC  230   a  attempt to change settings on the local controller  290 , the local controller  290  complies with the changes. 
     In various embodiments, the system  100  enters the CONFIGURATION mode or the VERIFICATION mode described previously. In an embodiment the system  100  may only enter the CONFIGURATION mode when the CF0 flag is reset (0) for all native SCs  230  on the subnet. A non-native SC  230  may enter the CONFIGURATION mode when either its CF0 bit or the CF1 bit is reset. As used herein a native SC  230  is an SC  230  that was present in the subnet during the most recent subnet configuration. A non-native SC  230  is an SC  230  that was not present, and was this not detected and is not remembered by other instances of the SC  230  in the subnet. As described above, the CF1 flag is set when it recognizes a configured indoor unit on its subnet  200 . If these conditions for entering the CONFIGURATION mode are not present, the system  100  may be placed in the VERIFICATION mode by a SC  230  on the subnet  200 . 
     If a Bit Error is detected when sending the startup message, e.g., DEVICE_Startup, the message is resent after a predetermined delay time in various embodiments. The delay time may be computed by an algorithm that employs the Device Designator. In one embodiment, the Device Designator field is parsed into 4-bit portions, each being a contiguous subset of bits of the Device Designator. If the Device Designator field is 32 bits, e.g., eight successive portions are thereby obtained. For brevity the bits of the Device Designator field are represented as DD[ 0 ]-DD[ 31 ]. In an example, the value of each 4-bit portion is incremented by 1, with the result being multiplied by 4 ms to determine a delay time associated with that portion. In an embodiment, the eight successive portions are associated with delay times as indicated below: 
     DD[ 0 ]-DD[ 3 ]: Startup Delay 
     DD[ 4 ]-DD[ 7 ]: First Resend Delay 
     DD[ 8 ]-DD[ 11 ]: Second Resend Delay 
     DD[ 12 ]-DD[ 15 ]: Third Resend Delay 
     DD[ 16 ]-DD[ 19 ]: Fourth Resend Delay 
     DD[ 20 ]-DD[ 23 ]: Fifth Resend Delay 
     DD[ 24 ]-DD[ 28 ]: Sixth Resend Delay 
     DD[ 29 ]-DD[ 31 ]: Seventh Resend Delay 
     If the message is not successfully sent after the eight attempts, subsequent delivery attempts may continue to be made repeating the eight resend delays. In some cases, the message may be resent up to a predetermined maximum, e.g. 255. If the message is not successfully sent within the predetermined maximum, the local controller  290  may be configured to disengage from the subnet  200 , e.g. enter a passive state. The local controller  290  may further be configured to execute the message send/retry cycle again after a predetermined delay period, e.g., about 5 minutes. 
     As described previously with respect to  FIG. 5 , in some embodiments a single physical device may include multiple logical devices. In cases in which a physical device contains more than one logical device, it may be preferable to limit all logical devices to be configured to the same subnet  200 . Generally each logical device, e.g., the aSC  470 , the user interface  480  and the comfort sensor  490 , sends out its own DEVICE_Startup messages. 
     Generally, logical devices are configured separately by messages sent by the aSC  230   a . In the case of a system device  410  that includes multiple logical devices, the aSC  230   a  assigns the same Subnet Identifier to each logical device. Taking the thermostat  590  ( FIG. 5 ) as an example, the aSC  230   a  may assign the Equipment Type and the Subnet Identifier to the aSC  510 . The aSC  230   a  may then also assign the same Subnet Identifier to the user interface  520  and the comfort sensor  530  via instances of an assignment message, e.g., aSC_DEVICE_Assignment. Each logical device may also respond with its own message acknowledging the assignment message, e.g., a DEVICE_Assignment_Acknowledge message. 
     As described previously, each local controller  290  may have an associated NVM, e.g., the NVM  320  ( FIG. 3 ). In some cases, the NVM may become corrupted. In various embodiments the contents of the NVM of each local controller  290  are archived by each SC  230  on the subnet of the network  200 . Each SC  230  may also archive the last active, inactive and disabled state of each local controller  290 . The contents of a corrupted NVM may then be restored using archival copies of the contents stored on any SC  230 . Additionally, in some cases a local controller  290  may archive application-specific data on one or more of the SCs  230 . For example, a local controller  290  may have associated data values that represent special parameters. During the COMMISSIONING state, the local controller  290  may archive the parameters on the one or more SCs  230 . Then, in various embodiments the SC  230  is configured to restore the contents of the NVM on a local controller  290  that has determined the contents thereof are corrupt. 
     In some embodiments a local controller  290  maintains a local copy of the NVM data. The local controller  290  so configured may recover its NVM data without intervention from an SC  230 . The local controller  290  may be configured to restore the contents of its NVM without changing its apparent behavior to other local controllers  290  in the system  100 . The local controller  290  may be further configured to verify the integrity of the NVM contents before sending a DEVICE_Startup message. 
     In an embodiment, when participation by an SC  230  is needed to recover NVM data, the recovery process may be performed by the device itself in conjunction with the aSC  230   a , Four example failure modes are described without limitation to demonstrate various features of the embodiment. 
     In a first case, the data stored on the local controller  290  NVM is corrupt, but a locally archived copy is valid. In this case, the device may recover the data from its internal backup in a manner that does not affect its apparent operation as viewed by the other local controllers  290 . In an advantageous embodiment, no indication is given to the other local controllers  290 , and control of the affected local controller  290  is unaffected. 
     In a second case, the data stored on the local controller  290  NVM is corrupt, but a locally archived copy is not valid, or no copy is locally stored. However, the aSC  230   a  stores correct values for the device. In this case, the local controller  290  may send a message, e.g., the DEVICE_Startup message, sent on Subnet  0 , using the default Equipment Type for that local controller  290 , with the CF6 flag cleared. It responds to all SC_Coordinator messages using the same message until a new Equipment Type and Subnet ID are assigned to it. As long as the NVM data are not recovered the CF6 flag remains reset. Once an aSC  230   a  takes over, it proceeds to assign the Equipment Type and Subnet ID to the local controller  290  as usual, which the local controller  290  stores internally. The aSC  230   a  recognizes the local controller  290  using its Device Designator and may assign the same Equipment Type and Subnet ID as previously assigned thereto. The local controller  290  may initially restore NVM data to default values stored in the device flash. The aSC  230   a  may in parallel enter the COMMISSIONING state to reprogram the local controller  290  with the data from its backup. The local controller  290  will typically replace any default values it may have placed in the NVM with data provided by the aSC  230   a.    
     In a third case, the archival data stored on the aSC  230   a  is corrupt. In this case, the aSC  230   a  may enter the VERIFICATION mode. In this state, the aSC  230   a  may obtain all data from associated local controller  290  as is normally obtained during verification. In some embodiments, the aSC  230   a  may instruct the local controller  290  to provide more data than is normally provided during the verification. 
     Finally, in a fourth case, the data stored on the local controller  290  and the archival data stored on the aSC  230   a  is corrupt. In this case the local controller  290  may restore the NVM to default values. The aSC  230   a  may obtain the default data as described for the third case. 
     Turning now to  FIG. 14 , illustrated is a method, generally denoted  1400 , of an algorithm that may be employed by the aSC  230   a  to assign the Equipment Type to such a local controller  290 . The method  1400  is representative of cases in which a device has an Equipment Type unknown to the aSC  230   a.    
     In step  1405 , the aSC  230   a  receives a startup message, e.g. DEVICE_Startup, from a local controller  290  having an unknown Equipment Type. In a branching step  1410 , the aSC  230   a  determines if another unknown local controller  290 , that has the same Equipment Type as the current unknown Equipment Type, has previously sent a startup message. If not, the method  1400  advances to a step  1415 . In the step  1415 , the aSC  230   a  assigns to the local controller  290  the Equipment Type provided by the local controller  290  in its startup message, and then ends with a step  1420 . 
     If in the step  1410  the aSC  230   a  determines in the affirmative, then the method  1400  advances to a step  1425 . In the step  1425 , a variable startET is set equal to the value of the Equipment Type received from the unknown local controller  290  in the startup message. A variable newET is set equal to the value of startET. A variable Increment is set to +1. The method  1400  advances to a step  1430 , in which the value of Increment, presently +1, is added to newET. 
     In a decisional step  1435 , if it is determined that there is another local controller  290  that has already been assigned the Equipment Type value currently stored by newET, the method returns to the step  1430 , where newET is again incremented. If instead it is determined in the step  1435  that there is not a local controller  290  with the Equipment Type held by newET, the method  1400  advances to a step  1440 , in which the aSC  230   a  assigns the value of newET to the Equipment Type of the unknown local controller  290 . 
     In a decisional step  1445 , the aSC  230   a  waits for an acknowledgement from the unknown local controller  290 , e.g., via a DEVICE_Assignment_Ack message. When the acknowledgement is received, the method  1400  advances to a decisional step  1450 , in which it is determined whether the assignment was successful. If the assignment was successful, the method  1400  ends at the step  1420 . 
     If in the step  1450  it is determined that the assignment was not successful, the method  1400  advances to a decisional step  1455 . If it is determined that the Equipment Type was rejected as being too high, the method  1400  advances to a step  1460 , in which newET is set equal to startET and the value of Increment is set to −1. The method then returns to the step  1430 . 
     If instead in the step  1455  it is determined that the Equipment Type is not rejected as too high, the method  1400  advances to a decisional step  1465  where it is determined if the Equipment Type is rejected for being too low. If the Equipment Type is not rejected as being too low, this condition represents the case, e.g., that there is another device already assigned the Equipment Type value. The method  1400  returns to the  1430  where newET is again incremented. If, on the other hand, it is determined in the step  1465  that the Equipment Type was rejected for being too high, the method advances to a step  1470 . The step  1470  establishes that the maximum number of devices is present in the system  100 . The unknown local controller  290  is set to a SOFT_DISABLED state, and the method  1400  ends with the step  1420 . 
     In one advantageous embodiment, the disclosure provides for a method of replacing controls in an HVAC system. In some circumstances, a controller, e.g., the UI/G  250 , may need to be replaced in an installed and configured HVAC system, e.g., the system  100 . Manual configuration and calibration of the new controller by the installer would be time consuming and expensive to the user of the system  100 . 
     In an embodiment, settings for the SC  230  are provided by an archived copy by another SC  230  as described previously. Each subnet controller, e.g., the SC  230 , stores the Device Designator and equipment serial and part numbers for each device in the network, e.g., the network  200 . The Device Designator and equipment serial and part numbers of an original local controller  290  may be assigned and stored on the local controller  290  at a manufacturing or assembly facility, e.g. However, the equipment serial and part numbers may be left blank for a replacement local controller  290 . The missing equipment serial and part numbers and a set CF5 flag, as describe above, identify the replacement local controller  290  as such to the SCs  230  on the network  200 . The CF5 flag may be provided by the replacement local controller  290  via a DEVICE_Startup message, e.g. Thus, the aSC  230   a  may configure the replacement local controller  290  with all pertinent parameter values, as well as the equipment serial and part numbers, all previously archived from the replaced local controller  290 . This approach significantly simplifies the replacement of local controllers  290  on the network  200 . 
     In an example embodiment, the aSC  230   a  categorizes the replacement local controller  290  based on the Device Designator actually stored thereon, rather than based on the archived Device Designator of the replaced local controller  290 . The aSC  230   a  determines that the replacement local controller  290  is a replacement part by the presence of the set CF5 flag, as described previously, and the lack of a local controller  290  on the subnet  200  that corresponds to the replacement local controller  290 . In the VERIFICATION mode, the replacement local controller  290  is placed in a SOFT_DISABLED state. The configuration of the replacement local controller  290  with the archived data from the replaced local controller  290  is performed during the COMMISSIONING state. Optionally, an alarm may be generated by the aSC  230   a  indicating that the replaced local controller  290  is missing. 
     In an embodiment, during the COMMISSIONING state the aSC  230   a  may verify that the replacement local controller  290  is compatible with the replaced local controller  290  with the participation of a user or installer. For example, the aSC  230   a  may prompt the user to automatically configure the replacement local controller  290  by listing a set of equipment serial and part numbers for each of the replacement local controller  290  and the replaced local controller  290 . The user may then be prompted to copy the archived values of all data, including all pertinent parameters and the equipment serial and part numbers onto the replacement local controller  290 . If the user accepts, then the configuration data are automatically copied to the replacement local controller  290 . In another embodiment, the user declines to automatically overwrite the configuration data of the replacement local controller  290 , and may enter desired configuration data via a UI/G  250 . 
     When a new local controller  290  is added to the subnet of the network  200 , this condition may be determined by the aSC  230   a  by the presence of a reset CF5 flag, and a Device Designator that does not match a local controller  290  already present on the subnet. In such a case, the equipment serial and part numbers are undisturbed in the COMMISSIONING state. However, as before the new local controller  290  may be placed in the SOFT_DISABLED state in the VERIFICATION mode. The CF5 flag may be protected against casual change. In some cases, the CF5 flag may only be changed in a privileged mode, e.g., an OEM test. 
     In various embodiments of the system  100  normal operation involves the delivery of DEVICE_Status messages and service demand messages by the aSC  230   a . A demand message may be expressed in terms of a percent of a full capacity of a demand unit  155 . A staged demand unit  155  may round off a percent of demand communicated to it to a value associated with a nearest stage capacity. In some embodiment the aSC  230   a  is configured to know values associated with the stages of a particular staged demand unit  155 , and may provide demand messages consistent with these values. In some embodiments a demand message targeting a demand unit  155  that includes a blower or similar device contain a blower override value. The demand unit  155  may change a blower speed from a default value associated with the requested demand level in response to the override value. An override value of 0 may indicate that the default may be used. 
     In some embodiments, a heating demand is mutually exclusive of a cooling demand. In cases of simultaneous demands that are not prohibited, a blower speed may default to a highest CFM value of the demands associated with the multiple demands. In one example, the configuration of the system  100  is changed from cool plus blower to blower only. A blower demand message may be sent with a desired blower level, followed by a cooling demand message that causes a compressor to cease operation. 
     In various embodiments, the aSC  230   a  tracks the availability of capacity of the demand units  155 . If for some reason a service, e.g., cooling, provided by a demand unit  155  becomes unavailable, the aSC  230   a  may clear demand messages that request that service. 
     Each local controller  290  is configured to transmit its own status on the RSBus  180  using the DEVICE_Status message. In various embodiments all DEVICE_Status messages share the same first two bytes regardless of equipment type. These two bytes may contain alarm and service status information. Each bit in the service status byte (service bits) will be ‘1’ if the service provided by the demand unit  155  associated with the local controller  290  is available, or if the local controller  290  sending the message does not know status of the service. The following device status table illustrates the principle: 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Device 
                 Fan 
                 Gas Heat 
                 Electric Heat 
                 Heat Pump Heat 
                 Cooling 
                 Humidification 
                 Dehumidification 
               
               
                   
               
             
            
               
                 Comfort Sensor 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Furnace 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Heat Pump 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Dehumidifier 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Logical AND 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     Each row of the table represents a device status vector maintained by the corresponding system device. Each column of the table represents a potential service provided by the corresponding system device. A potential service is a service that may be provided by the system  100  when the system  100  is appropriately configured. The system  100  need not actually be configured to provide the service. Also, each system device typically only provides a subset of the potential services, and may only provide a single service. 
     If the service is not available the bits of the service status byte are set to ‘0’. The aSC  230   a  receives the service bytes from all the various system devices  410  on the subnet  200  and performs a logical AND (any device sending a ‘0’ will result in the service being unavailable). In an embodiment, each alarm associated with a status bit modifies the status bit when the alarm is active. Thus, for example, an alarm condition of the furnace  120  may result in the associated status bit of the furnace  120  being set to “0” to indicate the furnace is unavailable. The following device status table illustrates the principle: 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Device 
                 Fan 
                 Gas Heat 
                 Electric Heat 
                 Heat Pump Heat 
                 Cooling 
                 Humidification 
                 Dehumidification 
               
               
                   
               
             
            
               
                 Comfort Sensor 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Furnace 
                 1 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Heat Pump 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Dehumidifier 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Logical AND 
                 1 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     Each bit that indicates the unavailability of a service, e.g. “0”, may be reset to a state indicating the service is available, e.g., “1” when an alarm condition related the unavailable service clears. The alarm may clear after the expiration of a predetermined interval, e.g. an “alarm timeout”, or the alarm may clear if reset by intervention of an operator, e.g., via the UI  240 . 
     This method advantageously simplifies maintenance of the system  100  by rendering it unnecessary to modify the device status in many cases when a system device is replaced. The method also eases system expansion by the manufacturer. 
     Each alarm may be an event-type alarm or a continuous-type alarm. An event-type alarm has a timeout associated with it, while a continuous alarm is active as long as the alarm condition persists. 
     The aSC  230   a  may then transmit its DEVICE_Status message including the combined results of all other local controllers  290  and the service byte of the aSC  230   a . The aSC  230   a  may then stop the demand corresponding to the service bit set to ‘0’. The demand may not be restarted until all devices restore the service bit and the resulting AND is equal to ‘1’. The demands from the same demand group (e.g. heating) can be substituted. In an embodiment if a heat pump service is not available and the system requires heating, gas heating or auxiliary electric heating may be used instead. In such a case, the aSC  230   a  may issue appropriate gas heating or electric heating demands. 
     The service bits may be set in all DEVICE_Status messages in all possible device states. In an embodiment of a routine VERIFICATION mode startup, the service bits are published by a particular local controller  290  on the RSbus  180  upon receipt by that local controller  290  of the first aSC_Change_State message after reset. Alternatively, the service bits may be published upon receipt by the publishing local controller  290  of an aSC_Assignment message after an asynchronous device reset. The service bits may be continuously updated to match the state of the service as determined by the local controller  290 . 
       FIG. 15  illustrates without limitation a method generally designated  1500  that is illustrative of a dialog between the aSC  230   a  and a demand unit  155 , e.g., an Integrated Furnace Control (IFC) or an Air Handler Control (AHC). Command messages are represented by underlined text. The method  1500  should not be considered a programming model or all-inclusive, but only as example to illustrate various principles of the disclosure. 
     The method  1500  begins with a step  1510 . In a step  1520  the aSC  230   a  determines if blower service is needed. If yes, the method advances to a step  1530 , in which the aSC  230   a  determines if blower service is available. If the blower service is available, the method advances to a step  1540 . In the step  1540  the aSC  230   a  sends a Blower_Demand message to the IFC or AHC, as appropriate. The method  1500  then advances to a step  1550 . In the step  1550 , the IFC or AHC transmits a DEVICE_Status message to the aSC  230   a  that includes the status of the blower. In a step  1560  the aSC  230   a  then sends a SC_UI_Zone_Status message to the UI  240  to provide feedback to the user/operator. If in the step  1530  the aSC  230   a  determines that blower service is not available, the method  1500  advances directly to the step  1550  without issuing a Blower_Demand message. The method  1500  ends with a step  1570 . 
     Messages between any UI  240  and the aSC  230   a  may be sent as a Class 1 message. In various embodiments Class 1 messages have priority over other messages, thus potentially reducing message latency. In most cases a display screen of the UI  240  is not updated with data directly from user input, but with data returned from the aSC  230   a  in response to the messages generated by the UI  240  in response to the user input. Exceptions to this general rule include cases in which a user selection results in altered equipment operation, such as a mode change. In that case, the user input may be buffered at the UI  240  until the selection is finalized, which may be either explicit or by timeout. Once the user selection is finalized, the UI  240  may send a message to the aSC  230   a  in response to the selection. 
     Local controllers  290  may be configured to promptly reply to a demand message. In an example embodiment, the local controller  290  acknowledges receipt of the demand message within about 100 ms by broadcasting a DEVICE_Status message. The DEVICE_Status message may have the Acknowledge bits set to 01b (ACK) if the message is positively acknowledged. Other aspects of the message may be otherwise unchanged from the case that no acknowledgment is made. A 0% demand is typically acknowledged in the same manner as non-zero demands. For a staged demand unit  155 , a demand below its minimum range may be treated as a 0% demand. In general, a demand message above 100% may be treated as a 100% demand. 
     Turning now to  FIG. 16 , illustrated is a method of the disclosure generally designated  1600  of manufacturing a subnet controller of an HVAC data processing and communication network. The method  1600  begins with an entry state  1610 . In a step  1620 , a bus interface device, e.g., the local controller  290 , is configured to receive a message from a subnet controller over the network. The subnet controller may be, e.g., the aSC  230   a . In a step  1630 , the bus interface device is configured to control a demand unit in response to the message. The method  1600  ends with an exit state  1640 . 
       FIG. 17  illustrates another method of the disclosure generally designated  1700  of a method of manufacturing a bus interface device networkable in an HVAC data processing and communication network. The method  1700  begins with an entry state  1710 . In a step  1720 , a physical layer interface, e.g., the PLI  310 , is configured to interface to a data network, e.g., the RSBus  180 . The physical layer interface may be located, e.g., on an active subnet controller such as the aSC  230   a . In a step  1730  a communication module, e.g. the communication module  340 , is configured to send and receive messages over the data network via the physical layer interface. The communication module may be located, e.g., on a bus interface local controller  290 . In a step  1740  a functional block, e.g., the functional block  350 , is configured to reset in response to a message received by the communication module. The functional block may be located on the same bus interface device as the communication module. The method  1700  ends with an exit state  1750 . 
       FIG. 18  illustrates another method of the disclosure generally designated  1800  of a method of manufacturing a subnet controller of an HVAC data processing and communication network. The method  1800  begins with an entry state  1810 . In a step  1820 , a physical layer interface, e.g., the PLI  310 , is configured to electrically interface to the network. In a step  1830 , a communication module, e.g., the communication module  340 , is configured to send and receive messages over the network via the physical layer interface. In a step  1840 , a functional block, e.g., the functional block  350 , is configured to respond to a message received by the communication module. The functional block thereby enters a disabled state in which the functional block does not execute control functions, but the communication module may receive messages over the network. The method  1800  ends with an exit state  1850 . 
       FIG. 19  illustrates a method generally designated  1900  of manufacturing a device networkable in an HVAC data processing and communication network. The method  1900  begins with an entry state  1910 . In a step  1920 , a physical layer interface is configured to interface to a network. The physical layer interface may be, e.g., the PLI  310 . In a step  1930 , a communication module, e.g. the communication module  340 , is configured to send and receive messages over the network via the physical layer interface. In a step  1940 , a non-volatile memory is configured to store configuration data. The non-volatile memory may be, e.g., the NVM  320 . In a step  1950 , a functional block, e.g., the functional block  350 , is configured to respond to a message received by the communication module thereby enabling a privileged operating mode not normally available to a user of the network. The method  1900  ends with an exit state  1960 . 
       FIG. 20  illustrates a method generally designated  2000  of manufacturing a device networkable in an HVAC data processing and communication network. The method  2000  begins with an entry state  2010 . In a step  2020 , a physical layer interface is configured to interface to the network. The physical layer interface may be, e.g., the PLI  310 . In a step  2030 , a communication module, e.g. the communication module  340 , is configured to send and receive messages over the network via the physical layer interface. In a step  2040 , a non-volatile memory, e.g., the NVM  320 , is configured to store configuration data. In a step  2050 , a plurality of logical devices is configured to be addressable via the communication module. Each logical device is thereby capable of being independently disabled. The method  2000  ends with an exit state  2060 . 
       FIG. 21  illustrates a method of manufacturing a device networkable in an HVAC data processing and communication network. The method  2100  begins with an entry state  2110 . In a step  2120 , a physical layer interface is configured to interface to a data network. The physical layer interface may be, e.g., the PLI  310 . In a step  2130 , a communication module, e.g., the communication module  340 , is configured to send and receive messages over the data network via the physical layer interface. In a step  2140 , a non-volatile memory, e.g., the NVM  320 , is configured to store device configuration data. The messages include a first class of messages that address the device using only a Device Designator of the device, and a second class of messages that address the device using a message ID formed from a portion of the Device Designator and an offset. The method  2100  ends with an exit state  2150 . 
       FIG. 22  illustrates a method of manufacturing an HVAC data processing and communication network. The method  2200  begins with an entry state  2210 . In a step  2220 , a first subnet controller, e.g., a first aSC  230   a , is placed in communication with a first bus interface device over a data bus. The bus interface device may be, e.g., the local controller  290 . In a step  2230 , a second subnet controller, e.g., a second aSC  230   a  or an iSC  230   i , is configured to archive configuration data of the first subnet controller and the bus interface device. The method  2200  ends with an exit state  2240 . 
       FIG. 23  illustrates a method of manufacturing an HVAC data processing and communication network. The method  2300  begins with an entry state  2310 . In a step  2320 , a demand unit is configured to provide a service having a maximum service capacity. In a step  2330 , a subnet controller is configured to send a message to the demand unit instructing the demand unit to provide a portion of the maximum. The method  2300  ends with an exit state  2340 . 
       FIG. 24  illustrates a method of manufacturing an HVAC data processing and communication network. The method  2400  begins with an entry state  2410 . In a step  2420 , a first subnet controller and a second subnet controller are configured to communicate over the network. In a step  2430 , the second subnet controller is configured to employ an arbitration algorithm to assert control over the network and the first subnet controller. The method  2400  ends with an exit state  2440 . 
     Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.