Abstract:
A vehicle intercom system allows conversation between individuals located, respectively, inside and outside of a vehicle. The voice units are linked by a vehicle controller area network eliminating the need for independent wiring of an intercom system.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an intercom system for motor vehicles and more particularly to an intercom system implemented over a vehicle&#39;s controller area network and providing the ability for occupants of the vehicle to talk to individuals outside of, but near to, the vehicle. 
   2. Description of the Problem 
   Medium duty trucks are often employed in local delivery service, as wreckers, as garbage collection vehicles and for other types of service, where laborers work in close proximity to the vehicle while it is moving. Under these circumstances there can be a need for a worker to talk or otherwise communicate with the driver from outside the vehicle. Communication can take the form of shouts and hand waving, which can be missed or easily misunderstood by a driver. 
   Intercom systems have been used to provide for voice communication between persons in different rooms of a building where direct interaction is inconvenient but there exists a need for quick and easy conversation. Such systems are also found on aircraft. Most such intercom systems have used a wiring system dedicated to the use of the intercom system. A disadvantage of applying such systems to trucks would be the need to provide wiring. Wiring for power and control functions on a vehicle has been implemented using a vehicle harness which bundles the wires together in pre-measured lengths to simplify building the vehicle. Adding wiring to the harness requires modification of the harness, increasing the harness&#39;s complexity and cost. Installing a wiring system outside of the harness defeats the purpose of having a harness and could require modification of the vehicle to take additional mounting points and to provide routing for the wire. 
   However, contemporary vehicles increasingly rely on computer networks to handle vehicle control functions. Inputs from dash controls are routed from the dash board to controllers and digital data are exchanged between component vehicle controllers such as an engine controller and a transmission controller over a bus. Vehicle data networks are well known in the art and are described in standards published by the Society of Automotive Engineers (SAE) as part of the SAE J1708 and J1939 standards. 
   The SAE J1939 standard is a specialized application of a controller area network (CAN) to motor vehicles. The SAE J1939 standard provides an open protocol defining formats for particular types of messages, but also allows for development of proprietary message types. A message on a controller area network is not routed to a node based on a node address, but rather broadcast to all of the nodes connected to the network. Every CAN message includes a priority and an indication of the message contents. The remaining nodes on the system determine in accord with their own programming whether to act on a message or discard it. Data transmission is serial. Priority determines control of the network in cases of contention in access. Messages are small, at most eight bytes, and are protected by checksum error detection. 
   Use of a network to route control signals promises greater physical simplicity through displacing much of the vehicle wiring harness, reducing manufacturing costs, facilitating vehicle electrical load management, and enhancing system reliability. Serial data bus communication is effective for eliminating dedicated communication paths between the numerous switches, sensors, devices and gauges installed on the vehicles, thus saving weight and reducing the complexity of service and manufacture. Such networks offer the potential of new and unforeseen vehicle functionality. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a vehicle intercom system. 
   It is another object of the invention to implement a vehicle intercom system which does not add to the physical complexity of the vehicle&#39;s electrical systems. 
   It is another object of the invention to implement an intercom system over a controller area network. 
   The invention provides an intercom system for communication between different areas of a vehicle. The intercom system comprises a bus for carrying data packets, the data packets including a priority indicator, a function identifier, a source identifier and a standard length data segment. A plurality of processor controlled modules are in communication with the bus, each of the processor controlled modules having a unique source identifier. Each of the processor controlled modules is also connected to the bus for receiving and interpreting data packets carried on the bus and for formatting and transmitting data packets over the bus. First and second intercom interfaces are positioned at mutually remote locations on the vehicle, the first and second intercom modules being further connected to first and second processor controlled modules. The first and second processor controlled modules further include signal processors for converting analog signals from the intercom interfaces to digital signals, bundling the digital signals into data packets and transmitting the data packets over the bus, and for recovering data packets having a function indication of an audio signal, converting data from the recovered data packets to an analog signal and applying the analog signal to the intercom interfaces. 
   Additional effects, features and advantages will be apparent in the written description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a medium duty truck illustrating the positioning of two intercom units of an intercom system on the truck; 
       FIG. 2  is a block diagram of a vehicle controller area network used to implement the invention; and 
       FIG. 3  is a high level circuit schematic of the intercom system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates in perspective view a medium duty truck  10 . Truck  10  includes a forward operator&#39;s cabin  12  and a rear utility section  16  which is depicted as a conventional cargo box, but which may be tow gear for a wrecker, compartmentalized garbage handling equipment, or other things. Mounted to exterior  18  of truck  10  is an intercom interface unit  20 . Another intercom interface unit  14 , possibly incorporated in a vehicle audio entertainment system, is located on dash  22  of forward operator&#39;s cabin  12 . An individual located in forward operator&#39;s cabin  12  may converse with an individual located near the rear of truck  10  over the intercom interface units  14  and  22 . 
   Referring now to  FIG. 2 , tractor  12  includes a electrical control system  25  based on an electrical system controller (ESC)  30  and three distinct networks including a first CAN network  27  based on the SAE J1939 standard and using the publicly defined message protocols to communicate with several drive train component controllers. The drive train component controllers include an electronic gauge cluster controller  32 , an automatic transmission controller  34 , an engine controller  36  and an anti-lock brake system controller (ABS)  38 . The several controllers  30 ,  32 ,  34 ,  36  and  38  are also nodes of the first CAN network  27 . A second CAN network  29  based on the J1939 standard uses manufacturer defined message codes and provides the data network over which the intercom system of the present invention is preferably implemented. Among nodes which are attached to the second CAN network  29  are a remote intercom module  40 , a remote power module  42  and the electrical system controller  30 . An intercom unit  44  is connected to remote intercom module  40 . Finally the vehicle may also include a low speed SAE J1708 compatible network  31  over which electrical system controller  30  can interrogate the status of a plurality of switches in a switch pack  46 . Electrical system controller directly handles a vehicle audio system  48  which in a preferred embodiment of the invention can serve, with the addition of a microphone, as an intercom station for the operator&#39;s cabin. 
     FIG. 3  is a high level circuit schematic of the components of the intercom system. Electrical system controller  30  is modified for operation as a base for an intercom station by addition of connections to an audio system  60 . If digital to analog and analog to digital conversion of signals from audio system  60  is required it is executed by a digital signal processor  54  connected between ESC microprocessor  52  and audio system  60 . Audio system  60  is connected to drive a loudspeaker  62  which is part of the accessible intercom station or interface  14 . Audio system  60  is further connected to a microphone  64  which is also part of station  14 . Audio system  60  is also connected to a mute line  58  from microprocessor  52  which cuts off sound reproduction from sources other than DSP  54 . In this way operation of an entertainment system may be interrupted if the intercom is in use. The intercom system may be activated by a switch from switch pack  46  which is connected to a J1708 protocol communications interface circuit  50  in ESC  30 . Interface circuit  50  may be interrogated by microprocessor  52  for the status of each of the switches in switch pack  46 . ESC  30  includes a CAN interface circuit  56  which provides conventional mailbox and buffering functions, as well as transceiver and memory management functions, all of which are well understood in the art. The voice data packets to be transmitted on data bus  29  and those received off of data bus  29  are handled by the CAN interface circuit  56 . The packets include a priority indication, an arbitrary, unique header associated with voice data and a fixed length segment of digitized voice data. Packets will be transmitted in order and, since no switching for routing of the packets is required, the packets may be played in the order received without loss of intelligence. Intelligibility can be preserved at fairly low resolution to reduce bandwidth requirements on the network. The additional loading on the network represented by full duplex voice transmission should remain under 10% of available capacity. Since other data communications requirements of secondary or non-drive train CAN networks typically represent a load of under 10% of available capacity, the total traffic load is such that minimal buffering normally prevents gaps in the voice message. 
   Typically vehicle  10  is provided with two intercom stations. The second intercom station is managed by a remote intercom module  40  which communicates with electrical system controller  30  over private data link (J1939 bus)  29 . Remote intercom module  40  (RIM) includes a CAN interface circuit  70  which is connected to data link  29  and which handles data communication. CAN interface circuit  70  is connected to a microprocessor  68  which identifies messages received over data link  27  for action, particularly voice data messages. Remote intercom module  40  may be connected to a remote input switch  66 , which is part of the intercom station interface  20 . Switch  66  may be used to activate a microphone  86  in the station interface  20  or for turning the remote intercom module  40  on. 
   CAN nodes do not have addresses and accordingly a RIM  40  does not require an address. RIM address jumper  90  connected to microprocessor  68  is accordingly closed. Microprocessor  68  is connected to a series of subsidiary devices within RIM  40  including a series of FETs  76 A- 76 C. FET  78  may be used to activate a light  80  indicating that microphone  86  is active. FET  72  is connected to microprocessor  68  for powering intercom station base unit  82 . Base unit  82  is in turn connected to a microphone  84  and a loud speaker  86 , which form part of the intercom station interface  20 . A digital signal processor  74  is connected between intercom station base unit  82  and microprocessor  68 . DSP  74  is one possible facility for provided bi-directional analog to digital or digital to analog conversion. 
   The present invention enables a vehicle operator to communicate by voice with an individual outside of the vehicle or located in another part of the same vehicle. The intercom stations are linked by an existing CAN network and require only an extension of the data link cable to the desired location of the station. The resulting intercom system exhibits minimal physical complexity and high adaptability. 
   While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.