Abstract:
A system ( 10 ) conveys energy and communication between a master ( 16 ) and a plurality of slaves ( 18, 20 ), via a bus ( 14 ). A power/voltage transmitter ( 44 ) of the master ( 16 ) provides electrical energy, having a voltage, onto the bus ( 14 ) to power the slaves ( 18, 20 ). The power/voltage transmitter ( 44 ) adjusts voltage past a predetermined threshold to provide a message frame and modulating voltage during the message frame to convey messages to the slaves ( 18,20 ). Voltage receivers ( 58, 64 ), at each slave ( 18, 20 ), detect the voltage modulations to discern messages from the master ( 16 ) during the message frame. Current transmitters ( 56, 62 ), at each slave ( 18, 20 ), modulate current during the message frame to convey messages to the master ( 16 ). The current transmitters ( 18, 20 ) utilize the modulation of voltage of the electrical energy to clock modulation of current. A current receiver ( 48 ) of the master ( 16 ) detects current modulations to discern messages from the slaves ( 18, 20 ). Preferably, the system is part of an occupant protection system ( 12 ), and the slaves ( 18, 20 ) include an occupant protection device.

Description:
TECHNICAL FIELD 
     The present invention is generally directed to an energy distribution and communication system and method of a vehicle occupant protection system. 
     BACKGROUND OF THE INVENTION 
     As the sophistication of vehicle occupant protection systems has increased, the number and complexity of vehicle occupant protection devices within the protection systems has increased. In response to the increased number of devices, there has been a movement toward centralized control of the devices within the protection systems to reduce cost and increase reliability. This change in the design approach for protection systems has brought about a need to design new arrangements for power distribution and data communication between a central controller and the devices. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect, the present invention provides an energy distribution and communication system between a central unit and a plurality of remote units. An electrical conductor interconnects the central unit and the remote unit for conducting electrical energy. Energy supply means provides electrical energy, which has a voltage, onto the conductor to power the remote units. Voltage messaging means, associated with the central unit, adjusts voltage of the electrical energy past a predetermined threshold to provide a message frame and modulates voltage of the electrical energy during the message frame to convey messages from the central unit to the remote units via the conductor. Voltage receiver means, at each remote unit, monitors the voltage of the electrical energy during the message frame and detects the voltage modulations to discern messages from the central unit. Current messaging means, at each remote unit, modulates current of the electrical energy during the message frame to convey messages to the central unit via the conductor. The current messaging means includes means for utilizing the modulation of voltage of the electric energy to clock current modulation. Current receiver means, associated with the central unit, detects current modulations to discern messages from the remote units. 
     In accordance with another aspect, the present invention provides an occupant protection system for protecting a vehicle occupant. An actuatable occupant protection device protects the vehicle occupant. A central unit controls actuation of the protection device and provides power for use by the protection device. The protection device is located remote from the central unit. An electrical conductor interconnects the central unit and the protection device for conducting electrical energy. The central unit includes energy supply means for providing electrical energy, having a voltage, onto the conductor. The central unit includes voltage messaging means for adjusting voltage of the electrical energy past a predetermined threshold to provide a message frame and for modulating voltage of the electrical energy during the message frame to convey messages from the central unit to the protection device via the conductor. The protection device includes voltage receiver means for monitoring the voltage of the electrical energy during the message frame and for detecting the voltage modulations to discern messages from the central unit. The protection device includes current messaging means for modulating current of the electrical energy during the message frame to convey messages to the central unit via the conductor. The current messaging means includes means for utilizing the modulation of voltage of the electric energy to clock current modulation. The central unit includes current receiver means for detecting current modulations to discern messages from the protection device. 
     In accordance with another aspect, the present invention provides a method of distributing energy and communicating between a central unit and a plurality of remote units. Electrical energy, having a voltage, is provided onto a conductor from the central unit to power the remote units. The voltage of the electrical energy is adjusted past a predetermined threshold to provide a message frame. The voltage of the electrical energy is modulated during the message frame to convey messages from the central unit to the remote units. The voltage modulations are detected at the remote units to discern messages from the central unit. Current is modulated during the message frame to convey messages from the remote units to the central unit. The current modulations are detected at the central unit to discern messages from the remote units. The step of modulating current includes utilizing the modulation of the voltage to clock current modulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will become apparent to one skilled in the art upon consideration of the following description of the invention and the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a vehicle occupant protection system having an energy distribution and communication system in accordance with the present invention; 
     FIG. 2 is an illustration of plots showing a voltage mode message and a current mode message that occur simultaneously on a communication bus of the communication system of FIG. 1; 
     FIG. 3 is a block diagram showing details of a master device of the communication system of FIG. 1; 
     FIG. 4 is a block diagram showing details of a hybrid master/slave device of the communication system of FIG. 1; 
     FIG. 5 is a block diagram showing details of a slave device of the communication system of FIG. 1; and 
     FIG. 6 is a plot illustrating an arbitration arrangement for voltage signals that occur simultaneously on the communication bus of the communication system of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     One representation of the present invention is schematically shown in FIG. 1 as an energy distribution and communication system  10  for a vehicle occupant protection system  12 . The occupant protection system  12  helps protect one or more vehicle occupant(s) in the event of a vehicle condition for which it is desired to protect the vehicle occupant(s). For example, the occupant protection system  12  helps protect the occupant(s) during a vehicle collision and during a vehicle rollover. Hereinafter, the occupant protection system  12  is referred to as the protection system  12 , and the energy distribution and communication system  10  is referred to as the communication system  10 , for brevity. 
     The protection system  12  is a distributed system, in that at least some of the components of the protection system are located remote from other components of the protection system. An energy transfer and communication bus  14  (hereinafter “the bus  14 ”) of the communication system  10  interconnects the components of the protection system  12 . The bus  14  includes at least one electrical conductor, such as a metal wire, along which electrical energy can flow to transfer electrical power and to convey messages. The components of the protection system  12  include a master device  16 , one or more hybrid master/slave device(s)  18 , and one or more slave device(s)  20 . The designations of “master” and “slave” are directed to the aspect of which components (e.g., masters) provide commands within the protection system  12 , and which components (e.g., slaves) are responsive to such commands. 
     The designations of “master” and “slave” also indicate how electrical energy is distributed within the protection system  12 . Specifically, the master  16  supplies electrical energy to power the hybrid master/slave(s)  18 , and the slave(s)  20 . The master  16  is connected to a source  22  of electrical energy, such as a battery of the vehicle, through suitable power regulation means, and is also connected to electrical ground  24  of the vehicle. The hybrid master/slave(s)  18  and the slave(s)  20  are not directly connected to the source  22  of electrical energy. Each hybrid master/slave  18  includes a power storage component  26  that is connected to the bus  14  to receive energy via the bus for use within the hybrid master/slave. Each slave  20  includes a power storage component  28  that is connected to the bus  14  to receive energy via the bus for use within the slave. 
     Turning now to the details of the components of the protection system  12 , the master  16  can be referred to as a central control unit. The master has a controller  30  with a processor, which executes a program (e.g., an algorithm), and/or with “hard-wired” circuitry to process information to make determinations for the protection system  12 . Commands from the master  16  that are intended for other components (e.g., the slave(s)) of the protection system  12  are based upon the determinations made within the controller  30 . 
     One function of the controller  30  is to process information indicative of vehicle operation and/or vehicle occupant characteristics to determine whether a need exists to provide protection to one or more vehicle occupants. The information indicative of vehicle operation and/or vehicle occupant characteristics is provided to the controller  30  via the communication system  10  and/or one or more sensor(s)  32  that are operatively connected to the controller. In one embodiment, the sensor(s)  32  include an acceleration sensor for detecting sudden vehicle deceleration such as would occur during a vehicle collision. 
     Another function of the controller  30  is to process information for diagnostic procedures within the protection system  12 . The information needed for the diagnostic procedures is provided to the controller  30  via the communication system  10  via commands from the master  16  for diagnostic information from the other components. 
     Each slave  20  performs a commanded function within the protection system  12 . Preferably, each slave  20  includes a controller  34  for handling messages and function component(s)  36  for performing commands conveyed via messages. It is to be appreciated that the controller  34  may merely be capable of recognizing addresses, responding to pre-defined messages, and issuing pre-defined messages. In other words, the controller  34  may be a “limited intelligence” component. 
     Preferably, at least one slave  20  is an actuatable occupant protection device. Each occupant protection device may be any suitable type of device. Examples of occupant protection devices include: an inflatable air bag device, an inflatable knee bolster device, an inflatable seat belt device, an inflatable headliner device, an inflatable side curtain device, a seat belt retractor lock device, a seat belt pretensioner device, and a D-ring height adjuster device. 
     It will be appreciated that upon the occurrence of a vehicle condition indicative of a situation in which a vehicle occupant is to be protected (e.g., a predetermined type of vehicle collision), the master  16  causes actuation of the occupant protection slave(s)  20  to help protect the occupant. In an example in which one slave  20  is an air bag module, the function components  40  include an air bag, a source of inflation fluid and a firing circuit. When actuated, the firing circuit causes the source of inflation fluid to inflate the air bag. 
     It is to be appreciated that some or all of the slave(s)  20  can be sensor devices. Further, such sensor slave(s)  20  could have “limited intelligence.” An example of a sensor slave includes an accelerometer. Hereinafter, the slave(s)  20  are referred to as the slave  20  (singular) for simplicity, but with the understanding that plural slaves may be present. 
     The hybrid master/slave(s)  18  are responsive to commands from the master  16 , but also provide commands to the slave  20 . Preferably, each hybrid master/slave  18  includes a controller  38  for handling messages and function component(s)  40  for performing actions (e.g., command requests). It is to be appreciated that the controller  38  may merely be capable of recognizing addresses, responding to pre-defined messages, and issuing pre-defined messages. In other words, the controller  38  may be a “limited intelligence” component. 
     The hybrid master/slave(s)  18  may include a sensor device and/or may also include an actuatable occupant protection device. Moreover, the hybrid master/slave(s)  18  may include certain determination making components, whose determination is utilized elsewhere within the protection system  12 . In addition, the hybrid master/slave(s)  18  may include actuatable components that provide indication of actuation and/or of the circumstances surrounding the actuation. Hereinafter, the hybrid master/slave(s)  18  are referred to as the hybrid master/slave  18  (singular) for simplicity, but with the understanding that plural hybrid master/slaves may be present. 
     In order for the master  16  to communicate and to supply power via the bus  14 , the communication system  10  includes a communication portion  42  that is part of the master. A power/voltage transmitter  44  of the communication portion  42  is connected to the electrical source  22  and ground  24 . The power/voltage transmitter  44  is also operatively connected to the controller  30  and to the bus  14 . One function of the power/voltage transmitter  44  is to provide electrical energy onto the bus  14  as a supply of electrical energy for the hybrid master/slave  18  and the slave  20 . A second function of the power/voltage transmitter  44  is to output voltage modulation signals onto the bus  14  to convey command messages from the master  16 . 
     The communication portion  42  of the master  16  includes a voltage receiver  46  that is operatively connected to the controller  30  and to the bus  14 . The voltage receiver  46  receives messages from the bus  14  that are conveyed via voltage modulation. Such voltage-modulation messages are placed onto the bus  14  by the hybrid master/slave  18 . 
     The communication portion  42  of the master  16  also includes a current receiver  48  that is operatively connected to the controller  30  and to the bus  14 . The current receiver  48  receives messages from the bus  14  that are conveyed via current modulation. Such current-modulation messages placed onto the bus  14  by the slave  20  or the hybrid master/slave  18 . Thus, it is to be noted that the communication portion  42  of the master  16  sends messages via voltage modulation, receives messages via voltage modulation, and receives messages via current modulation. 
     In order for the hybrid master/slave  18  to communicate, the communication system  10  includes a communication portion  52  that is part of the hybrid master/slave. The communication portion  52  of the hybrid master/slave  18  includes a voltage transmitter  54  operatively connected to the controller  38  and to the bus  14 . The voltage transmitter  54  modulates voltage on the bus  14  to send messages to the master  16  (or another hybrid master/slave). 
     A current transmitter  56  of the communication portion  52  is operatively connected to the controller  38  and to the bus  14 . The current transmitter  56  modulates current on the bus  14  to send messages to the master  16 . A voltage receiver  58  of the communication portion  52  is operatively connected to the controller  38  and to the bus  14 . The voltage receiver  58  receives voltage-modulated signals on the bus  14 . Such voltage-modulation messages are placed onto the bus  14  by the master  16  (or another hybrid master/slave  18 ). Thus, it is to be noted that the communication portion  52  of the hybrid master/slave  18  sends messages via voltage modulation, sends messages via current modulation, and receives messages via voltage modulation. 
     In order for the slave  20  to communicate, the communication system  10  includes a communication portion  60  that is part of the slave. A current transmitter  62  of the communication portion  60  is operatively connected to the controller  34  and to the bus  14 . The current transmitter  62  modulates current on the bus  14  to send messages to the master  16  (or to a hybrid master/slave  18 ). 
     A voltage receiver  64  of the communication portion  60  is operatively connected to the controller  34  and to the bus  14 . The voltage receiver  64  receives voltage-modulated signals on the bus  14 . Such voltage-modulation messages are placed onto the bus  14  by the master  16  or the hybrid master/slave  18 . Thus, it is to be noted that the communication portion  60  of the slave  20  sends messages via current modulation and receives messages via voltage modulation. 
     Turning now to the operation of the communication system  10 , when communication is not happening (e.g., communication signals are not being transmitted along the bus  14 ) the communication system “idles.” During idle, the power/voltage transmitter  44  of the master  16  provides electrical energy on the bus with a non-modulating voltage. The voltage of the electrical energy on the bus  14  during the idle period is referred to as an idle voltage. As shown in FIG. 2, the idle voltage is at a first predetermined level that is a relative high level. The power storage component  26  (FIG. 1) of the hybrid master/slave  18  and the power storage component  28  of the slave  20  accept and store electrical energy to power the components of the respective device. 
     Focusing on the communication periods (e.g., non-idle periods), the communication that occurs on the bus  14  can be full duplex or half duplex. Specifically, communication via voltage modulation (e.g., from the master  16  or the hybrid master/slave  18 ) and current modulation (e.g., from the slave  20  or the hybrid master/slave  18 ) can occur simultaneously. Preferably, communication via voltage modulation (e.g., from the master  16  or the hybrid master/slave  18 ) can occur without simultaneous current modulation communication. 
     The master  16  generally controls certain aspects with respect to the occurrence of a communication period on the bus  14 . Specifically, in order for communication to occur on the bus, the master  16  provides a message frame (see FIG.  2 ). The message frame is a portion of time in which the voltage that is provided onto the bus  14  is reduced from the value of the idle voltage to a level below a frame threshold value. 
     During the message frame time period, the voltage is modulated (e.g., by the master  16  or the hybrid master/slave  18 ) between a first level (e.g., a lower level to represent logic LOW) and a second level (e.g., a higher level to represent logic HIGH). Both the logic LOW voltage value and logic HIGH voltage value are below the frame threshold value. 
     The voltage-modulated message comprises a series of bit segments. Within each bit segment, the voltage is modulated to provide a pulse at the logic HIGH voltage value. Each bit segment is defined to end on a falling edge of a pulse. The duration of the pulse at the logic HIGH voltage value has either a first duration or a second duration. The first pulse duration represents a binary zero, and the second pulse duration represents binary one. Thus, the voltage mode of communication is via pulse-width-modulation (PWM). Further, the voltage mode communication is serially transmitted and digital. 
     The drop of the voltage from the idle voltage value to begin the message window signals all the devices (e.g., the slave  20  and the hybrid master/slave  18 ) that receive voltage signals that the idle period has ended and that a message is about to be placed on the bus  14 . Thus, minor voltage fluctuations about the idle voltage that may occur while the communication system  10  is in the idle mode are ignored by the components of the communication system. Such voltage fluctuation can occur via noise on the bus  14 . The signal-to-noise ratio of the communication is higher because the components of the protection system  12  only participate in communication while the voltage is below the frame threshold value. 
     The master  16  (FIG.  1 ), or the hybrid master/slave  18 , controls the length of the message frame. Thus, the number of bit segments is variable. Accordingly, the length of the messages is variable, and may be of any desired length. In other words, the message length may be changed for each message. Preferably, communication in the voltage mode is asynchronous in that the master  16  or hybrid master/slave  18  can transmit at will, regardless of whether current mode communication is occurring. 
     The master  16  controls the provision of the message frame. However, the hybrid master/slave  18  can cause the master  16  to provide a message frame such that the hybrid master/slave can transmit a voltage mode signal. It should be noted that the power/voltage transmitter  44  of the master  16  is current limited. Further, the voltage transmitter  54  of the hybrid master/slave  18  can pull down the voltage on the bus  14  (e.g., by shunting). The voltage receiver  46  of the master  16  senses the hybrid master/slave  18  pulling the voltage down and accordingly causes the voltage to drop, to provide the message frame for use by the hybrid master/slave (e.g., the master  16  lowers the current limit and the hybrid master/slave  18  controls the line voltage by changing shunt current). 
     Current signals are created via modulation of the amount of current on the bus  14  above/below a threshold value (see FIG.  2 ). For current mode messages from the hybrid master/slave  18 , the current transmitter  56  varies the amount of current flowing on the bus  14 . For current mode messages from the slave  20 , the current transmitter  62  varies the amount of current flowing on the bus  14 . Preferably, each of the current transmitters  56  and  62  includes a current sink device to vary the current draw. Preferably, current-mode communication only occurs during voltage-mode communication. 
     A sequence of data bit segments occurs during the current-mode communication, and each data bit has a binary value that is dependent upon the current draw value on the bus  14 . Specifically, a first range of current draw values (e.g., below the threshold value) is indicative of logic LOW and a second range of current draw values (e.g., above the threshold value) is indicative of logic HIGH. For each data bit segment, a binary zero is represented by logic LOW at a predetermined point within the data bit segment. A binary one is represented by logic HIGH at the predetermined point with the data bit segment. Thus, the current mode communication is digital and serial. 
     As noted above, the current mode communication occurs simultaneously with the voltage mode communication. Thus, the current mode communication occurs during the message frame. The current transmitters  56  and  62  within the hybrid master/slave  18  and the slave  20 , respectively, do not require an accurate internal clocking device to clock modulation of the current to provide the data bits of the current mode communication. Instead, the current transmitter relies upon the voltage data that is being simultaneously transmitted across the communications bus to clock the current modulation data. Specifically, current mode bit segments are defined by the falling voltage edges. The falling voltage edges occur at the beginning of the message frame (e.g., the fall from the idle voltage) and the falling edge of each pulse. Specifically, each current mode bit segment starts/ends when the voltage falls below a message threshold voltage value (FIG.  2 ). The use of the pulse width modulation of the voltage mode communication to clock the current mode communication results in automatic synchronization of the baud rate. 
     Each current transmitter  56 / 62  (of either the hybrid master/slave  18  or the slave  20 ) can change the data value being sent onto the bus  14  at each falling edge of the pulse width modulated voltage signal. At the current receiver  48  of the communication portion  42  of the master  16 , the current value on the bus  14  is latched by the current receiver at each falling edge of the modulated voltage signal. Accordingly, the current receiver  48  latches the current draw value just as the power/voltage transmitter  44  is about to switch to end the voltage-mode communication bit. 
     As a further aspect of the current modulation communication, the provision of each sequence of current-mode communication bits is done via a non-return to zero format. Accordingly, when each current transmitter  56 / 62  (e.g., in the hybrid master/slave  18  or the slave  20 ), is sending a plurality of sequential data bits that are the same (e.g., two or more data bits that are all binary one), the current transmitter can maintain the current draw on the communications bus at the value indicative of the binary value without returning to a neutral value or a zero value. This has the benefit of speeding communications along the bus  14 . 
     FIG. 3 illustrates an example of components within the master  16 . Specifically, a digital layer of the controller  30  has a plurality of inputs and outputs. A first series of outputs is provided to the power/voltage transmitter  44 . 
     A first output  68  is connected to a high current supply driver  70  of the power/voltage transmitter  44 . Preferably, the high current supply driver  70  includes an amplifier. A signal provided on the first output  68  is active when the communication system  10  is idling. As will be recalled, electrical energy is provided at a predetermined idle voltage level during idle. Accordingly, the output of the high current supply driver  70  provides the idle voltage. 
     A second output  72  of the digital layer of the controller  30  is connected as a control to a low current supply transmitter driver  74  of the power/voltage transmitter  44 . Preferably, the low current supply transmitter driver  74  includes an amplifier. A signal on the second output  72  is active when the master  16  or the hybrid master/slave  18  is transmitting a voltage mode signal. Thus, the low current supply transmitter driver  74  is only active or ON during voltage mode communication. 
     A third output  76  of the digital layer conveys a frame signal that sets the message threshold voltage level for the message frame. A fourth output  78  of the digital layer is a modulated signal that sets the logic low and logic high values for conveying the data bits. The third and fourth outputs are combined in a pre-driver circuit  80  and the output of the pre-driver circuit is provided as the input to the low current supply transmitter driver  74 . Accordingly, the output provided by the low current supply transmitter driver  74  is dependent upon the combined values for the frame and pulse-width modulated data signal. 
     The voltage receiver  46  is connected to the bus  14  at the same node as the output of the high current supply driver  70  and the output of the lower current supply transmitter driver  74 . The output of the voltage receiver  46  is provided as a first digital input to the digital layer of the controller  30 . The voltage receiver  46  includes any suitable structure for detecting the voltage on the bus  14  and providing a digital output signal that is indicative of the modulated voltage signal that is present on the bus. In one embodiment, the voltage receiver  46  includes a plurality of comparators and filters. 
     A current sense component  82  of the current receiver  48  is located on the bus  14 , and the two end nodes of the current sense component are connected to processing circuitry  84  of the current receiver. The processing circuitry  84  outputs a digital signal. The signal is a second input to the digital layer of the controller  30 , and is indicative of the amount of current on the bus  14 . Preferably, the processing circuitry  84  includes a comparator, an amplifier, and a filter. A return line  86  of the bus  14  is connected to the digital layer of the controller  30 . 
     FIG. 4 illustrates an example of components within the hybrid master/slave  18 . A diode  90  is connected between a node on the bus  14  and a power line  92  that extends to a digital layer of the controller  38 . A return line  94  is connected to the digital layer of the controller  38 . A capacitor  96  is connected between the power line  92  and the return line  94 . The diode  90  and the capacitor  96  form the energy storage component  26 . When the communication system  10  is idling, the capacitor  96  is charged. The energy stored in the capacitor  96  is used to power hybrid master/slave  18  when the communication system  10  is engaged in communication. The diode  90  prevents energy from the capacitor  96  from flowing back onto the bus  14 . 
     The digital layer of the controller  38  has a plurality of inputs and outputs. A first series of outputs is provided to the voltage transmitter  54 . A first output  100  of the digital layer conveys a frame signal. A second output  102  of the digital layer conveys a modulated signal that sets the logic low and logic high values for conveying the data bits. The first and second outputs are combined in a pre-driver circuit  104 , and the output of the pre-driver circuit is provided as the input to a high current sink transmitter driver  106 . Accordingly, the output provided by the high current sink transmitter driver  106  is dependent upon the combined values for the frame and pulse-width modulated data signal. Thus, the output of the voltage transmitter  54  is an indicator signal to the master  16  to lower the current limit for the lower voltage for the message frame, and a modulated voltage signal to convey the message. Specifically, the master changes the current limit. The hybrid master/slave  18  controls the line voltage by changing the sink current until the voltage reaches the desired state. 
     In the illustrated example, the current transmitter  56  includes a slave line driver low current sink  110 , which is connected along the bus  14 . A slave response control line  112  is connected between the digital layer of the controller  38  and the current sink  110 . The current sink  110  varies the amount of current on the bus  14  and is controlled via a slave response control signal from the digital layer. The slave response control signal is modulated in a sequence to provide current draw indicative of logic low and logic high. 
     The voltage receiver  58  is connected to the bus  14  at the same node as the output of the high current sink transmitter driver  106 . First and second output lines  114  and  116  from the voltage receiver  58  are connected to the digital layer of the controller  38 . A digital signal that is indicative of the message frame is provided via the line  114  as a first input to digital layer of the controller  38 . A digital signal that is indicative of the modulated data signal is provided via the second line  116  as a second input to the digital layer of the controller  38 . The voltage receiver  58  includes any suitable structure for detecting voltage on the bus  14  and providing the digital output signals indicative of the message frame and the modulated voltage. In one embodiment, the voltage receiver  46  includes a plurality of comparators and filters. 
     With regard to the master  16  (FIG. 3) and the hybrid/master slave  18  (FIG.  4 ), the example diagrams indicate that the digital layer of the respective controller may include an ASIC or a micro-controller performing a preprogrammed process, and many of the components connected to digital layer inputs/outputs (e.g., the components are not incorporated within the digital layer ASIC). It is to be appreciated that the master  16  and/or the hybrid master/slave  18  may be configured such that all of the components are within a single ASIC. 
     FIG. 5 illustrates an example of components within the slave  20 . A diode  120  is connected between a node on the bus and a power line  122  that extends to an ASIC  124  within the slave  20 . A return line  126  is connected to the ASIC  124 . A capacitor  128  is connected between the power line  122  and the return line  126 . The diode  120  and the capacitor  128  form the power storage component  28 . When the communication system  10  is idling, the capacitor  128  is charged. The energy stored in the capacitor  128  is used to power the slave  20  when the communication system  10  is engaged in communication. The diode  120  prevents energy from the capacitor  128  from flowing back onto the bus  14 . 
     A digital layer of the controller  34  may be part of the ASIC  124  and has an output and two inputs. A slave line driver current sink  130  of the current transmitter  62  is connected along the bus  14 . A slave response control line  132  is connected between the digital layer of the controller  34  and the current sink  130 . The current sink  130  varies the amount of current on the bus  14  and is controlled via a slave response control signal from the digital layer. The slave response control signal is modulated in a sequence to provide current draw indicative of logic low and logic high. 
     The voltage receiver  64  is connected to the bus  14 . A first output line  134  of the voltage receiver  64  is connected to the digital layer of the controller  34 . The first output line  134  conveys a digital signal that is indicative of the presence of the message frame. A second output line  136  of the voltage receiver  64  is connected to the digital layer of the controller  34 . The signal conveyed on the second output line  136  is a digital signal and is indicative of the modulated data signal. The voltage receiver  64  includes any suitable structure for detecting voltage on the bus  14  and providing the digital output signals indicative of the message frame and the modulated voltage. In one embodiment, the voltage receiver  64  includes a plurality of comparators and filters. 
     It should be noted that in the illustrated example, the slave ASIC  124  contains the voltage receiver  64 , the current transmitter  62 , and the digital layer of the controller  34 . It is to be appreciated that the ASIC  124  may be designed to only include the digital layer, with the voltage receiver  64  and current transmitter  62  located outside of the ASIC. 
     The present invention provides various additional aspects. For example, a zero dominant voltage based arbitration scheme can be used on the bus  14 . Specifically, it is possible that the master  16  and the hybrid master/slave  18  may attempt to communicate via a voltage-modulated signal at the same time. On the bus, any binary logic one that occurs simultaneously with a binary logic zero will be interpreted to be binary logic zero. This scheme is shown in FIG.  6 . 
     The master  16  and the hybrid master/slave  18  each monitor their own message. When the master  16  is transmitting and a bit does not match, the master immediately ceases transmission of its message. Similarly, when the hybrid master/slave  18  is transmitting and a bit does not match, the hybrid master/slave immediately ceases transmission of its message. This allows for non-destructive arbitration. No messages are lost. For example, if a first device (e.g., the master  16 ) was sending an address 0001 and a second device (e.g., the hybrid master/slave  18 ) was sending an address 0011, the message on the bus would be 0001 (e.g., the third bit of the message from the second device would drop to 0). The second device would recognize that its third bit did not match and would immediately cease transmission. The first device would complete its message without disruption. Other types of arbitration (e.g., non-address arbitration) can be used. 
     Also, it is to be appreciated that the master  16  is the overall master of all of the other devices including the hybrid master/slave  18 . If the hybrid master/slave  18  is transmitting a voltage signal at a time when the master  16  wishes to transmit a high priority voltage based message, the master causes the hybrid master/slave to cease transmission. Specifically, the master  16  stops a voltage-based message from the hybrid master/slave  18  by pulling the voltage on the bus  14  to ground. In response, the hybrid master/slave  18  ceases modulation of the voltage on the bus  14 . 
     Also, it is contemplated that slave current response arbitration may be included in the system. Also, it is contemplated that the only current mode response transmitted is from the previously addressed slave/hybrid master. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. For example, the communication system need not have slaves (e.g., only master/slaves) or need not have master/slave (e.g., only slaves). Also, the system could have multiple communication buses extending from the master. Each of the multiple buses could connect slaves only, master/slaves only, or a combination of slaves and master/slaves. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.