Powered communication link
BACKGROUND OF THE INVENTION 
     The following invention relates to a communication system in which bidirectional communication is provided over a conductive link from a central controller to a remote substation wherein the central controller also provides power over the conductive link to the remote substation. 
     In some applications for communications systems having a central data processing controller linked by a single pair electrical conductor to a remote data processing substation, it is sometimes necessary to provide the remote substation with electrical power over the conductor on a time-shared basis with the data. This may include applications in which the remote station is movable along a track or rail where the track provides the conductive link for data transmission and reception. Since it is in motion, the remote data processing substation may not have available to it a source of power, and it is therefore necessary in such cases to provide power to the remote substation along with data over the single pair conductive link. Also, at times, the single pair conductive link may be of the type which is prone to intermittent open circuits. This can occur when the conductive link is in the form of a rail or track, or where the link must pass through a slip ring where electrical continuity is maintained by a contact bearing against the slip ring or track. 
     One example of an environment to which the invention relates is in a communications controller located on the steering wheel of an automobile which communicates with a central console located underneath the dashboard. Systems have been proposed for automobiles in which a communications input and display for controlling such automobile functions as a radio, tape deck or cellular radio telephone would include push buttons or contact pads to control various functions related thereto. For example, functions for a tape deck such as &#34;play,&#34; &#34;fast forward,&#34; or &#34;reverse&#34; as well as volume and tone controls have been located on the steering wheel to control the operation of a tape deck mounted in its usual location. A system generally of this type may be obtained from Blaupunkt and is known as a Blaupunkt-Rinspeed Intelligent Steering Wheel. The Blaupunkt system includes push buttons to operate an AM/FM cassette receiver, a CD player and a cellular radio telephone. In this system, however, communication from the steering wheel to the central console is via infrared links where the infrared transmitters in the wheel are powered by batteries. This is a disadvantage because of the need to change the batteries and because the batteries may run out of charge at inopportune times. This could be especially troublesome with a cellular radio telephone. Also the infrared link is one way communication only, and the remote control on the wheel is incapable of receiving data from the console. 
     In most automobiles, the only conductive link that extends through the steering wheel column is the horn wire with the column itself supplying the second conductive portion of the wire pair. The integrity of the horn wire is maintained by a contact and slip ring arrangement so that the steering wheel may be turned through any arc and still have the horn available. The idea of utilizing the horn link as both a communication link and a means for powering the remote data terminal has not been pursued in the past for the application described above because, given the low voltage nature of data pulses, and given the noise generated by the slip ring contact, communication from the remote data and display substation to the central data processing controller has been impractical. The slip ring contact is so noisy that the receivers at either end of the link interpret noise spikes as data pulses. Also digital communication can easily be lost, because although the slip ring contact is sufficient to maintain the integrity of the horn signal, which is a relatively high voltage and high current signal, the link would appear to be an intermittent connection to the microprocessors that would be needed at either end. 
     Another example of the environment in which the invention may be found is in a printer having a print head which must move along a pair of metal tracks. The print head must be powered from the metal tracks and must also receive data along this link from the printer. Another example is a model railroad. It is desirable in the case of a model railroad to power the locomotive as well as send data through the track. In the past the locomotive has not been able to send data back to the central controller, which would be a desirable feature. This data could include speed or distance travelled. The problem with attempting any more than the most elementary form of analog communication in any of these cases is the noise generated by the sliding contact. 
     SUMMARY OF THE INVENTION 
     The problems mentioned above are solved by the apparatus of the present invention which comprises a powered communication link between at least two data substations linked by a single pair conductor wherein a power supply is located at a central controller for supplying electrical power over the single pair conductor to a second or remote substation along with receiver/transmitter circuitry that discriminates against the noise generated by a sliding contact. The remote substation includes an electrical power storage circuit for storing the electrical power supplied over the conductor and also includes a data entry and display terminal powered by the electrical storage circuit. A computer at the central controller periodically stops the application of electrical power to the conductive link and sends data to and receives data from the remote substation. The data transmission thus alternates with the transmission of power, time-sharing the conductive link. 
     The pulsed nature of the power transmission requires that the power pulses be stored as charge in a storage device, such as a capacitor, and then provided to the remote data substation in the form of a DC voltage. In one embodiment of the invention, a voltage sensing circuit senses the amount of voltage provided by the storage device and sends a signal over the link from the remote data substation to the central controller indicating that more power is needed if this voltage drops below a threshold. The controller will then respond by adapting either the frequency or the period of the power pulses transmitted to the substation. 
     Each end of the communication link includes a pull-up network which is operative upon any removal of power from the link for raising the potential of the link except in the presence of data, to prevent the false triggering of microprocessors located at both the remote substation and the central controller. This is necessary because the microprocessors at each end must be configured to accept a low level logic signal as the active signal in order to avoid confusion with the power pulses. When power is lost through the intermittent nature of the conductive link, it is important that the low level pulses produced at that time, which are actually noise, are not interpreted as data pulses by either microprocessor. 
     Both microprocessors include internally generated programs for controlling the communication protocol between central controller and remote substation. It is important that in the event of loss of communication, both microprocessors be capable of being reset to an initialized condition so that the program can start all over again without loss of data. Thus, the remote substation includes a timer for resetting its microprocessor in the event that no timing pulses are received within a preset period of time. 
     In some environments of the invention, particularly those in which the conductive link is the path for a signalling device such as a warning horn or light, a fail-safe switching circuit is provided which connects a switch which controls the signalling device directly to the conductive link in the event that communication between the remote substation and the central controller is lost. At all other times the status of the horn switch is communicated as a bit of information in the data transmission from the remote substation to the central controller. 
     It is the principal object of this invention to provide a powered two-way communication link between a central controller and a remote substation having no internal power source over a single pair conductor which may include an intermittent or noisy connection. 
     A further object of this invention is to provide a powered two-way data communication link over a single pair conductor in which power pulses and data pulses are multiplexed, and which includes an energy storage device at the remote substation for storing energy provided by the power pulses to provide the electrical power necessary to operate the substation. 
     A still further object of this invention is to provide a communication link over a single pair conductor between a central controller and a remote substation powered over the single pair conductor in which the energy level of a storage device at the remote substation is monitored and a signal is provided to the central controller indicating when more power is needed. 
     A still further object of this invention is to provide a powered communication link between a central controller and a remote substation in which the remote substation includes a signalling device and includes a fail safe circuit for connecting the signalling device with the communication link in the event of a failure of either the central controller or the remote substation. 
     The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of the system of the invention adapted for use in the steering column of a vehicle. 
     FIG. 2 is a block schematic diagram of the central controller shown in FIG. 1. 
     FIG. 3 is a block schematic diagram of the remote substation and data entry device shown in FIG. 1. 
     FIG. 4 is a detailed schematic diagram of a portion of the circuitry shown in the block schematic diagram of FIG. 3. 
     FIG. 5 is a detailed schematic diagram of a portion of the circuitry shown in the block schematic diagram of FIG. 2. 
     FIG. 6 is a schematic diagram of a watchdog timer circuit as shown in the block diagram of FIG. 2. 
     FIG. 7 is a waveform diagram illustrating how power and data are multiplexed over the communications link connecting the remote substation and the central controller. 
     FIG. 8 is a schematic diagram of the energy level detection circuit of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an automobile (not shown) includes a steering wheel 10 connected to a steering column 12. The steering column 12 includes a horn switch 14 which communicates with a remote substation 16. Connected to the remote substation 16 by way of a wiring harness 18 is a data entry device 20. The data entry device 20 could have many functions, some examples of which are controls for an AM/FM cassette receiver, CD player, and/or cellular radio telephone. The remote substation 16 has a ground wire 22 connected to the metal casing of the steering column 12. Data and power are supplied from a central controller 24 to the remote substation 16 over a single conductor 26 which may, for example, be the conductor that is ordinarily used as the horn wire. As is commonly the case, the horn wire 26 is connected to a slip ring 28 which makes contact with a rotary button 30 connected by wire to the remote substation 16. The central controller 24 is a microprocessor-based computer and receiver/transmitter which may control the various accessory devices described above and designated generally herein as 34. The central controller is also connected to the automobile horn 36. The ground path for the electrical circuit is the steering column 12. 
     The remote substation 16 is unpowered except from a power supply controlled by the central controller 24 and, as such, the conductor 26 and the ground path provided by the steering column are the only means for supplying power to the remote substation 16. In addition, the conductor 26 provides a two-way communication path between the remote substation 16 and the central controller 12 and may also provide the conductive link from the horn switch 14 to the horn 36. 
     Referring now to FIG. 2 the central controller 24 includes a computer 40 which comprises a microprocessor and associated memory (not separately shown). The computer 40 is connected to a link transmitter 42 which has an output coupled to the bottom portion (bottom link) 26a of conductive link 26. A link receiver 44 is also coupled between the bottom link 26a and the computer 40. A battery 46 which may be the automobile battery, provides a DC voltage for a power supply 48. The power supply 48 is, in turn, connected to a link power control circuit 50 which couples the power supply to the bottom link 26a under control of the computer 40. A false sync prevention circuit 52 is connected between the power supply 48 and the bottom link 26a, the purpose of which will be described in more detail below. The accessories are controlled by an accessory controller circuit 54 which is coupled by way of a communication interface 56 to the computer 40. The computer 40 also includes a watchdog timer circuit 58 which automatically resets the computer 40 in the event of a communications failure. If the invention is to be used in the environment of an automobile with the horn wire as the conductive link 26, a horn protection circuit 60 is provided as shown in dashed outline in FIG. 2. This is connected to a horn control circuit 62 which is also connected to computer 40. 
     The remote substation 16 is shown generally in FIG. 3. Like the central controller 24, the remote substation 16 includes a computer 64 which is connected to the conductive link by way of a transmitter 66 and a receiver 68. A synchronous power switch 70 at the end of the top portion (top link) 26b of the conductive link 26 controls a display regulator 72, a logic regulator 74 and provides power to an energy storage element 76. The logic regulator 74 is coupled to a false sync prevention circuit 78 which is in turn coupled to the top link 26b. In order to monitor the amount of energy present in the energy storage element 76, an energy level detection circuit 80 is provided which includes an output connected to the computer 64. Various devices such as the data entry device 20 may be connected to the computer 64 by way of a data communications interface 82. There may also be data output devices 84 which may include, for example, an LCD display, LEDs or an audio transducer which are connected to the computer 64 by way of a data output communication interface 86. The nature of the data entry devices 20, the data output devices 84, and also the interfaces connecting each of these respective devices to the computer 64 are determined by the application desired. Conventional or special purpose interfacing and choice of the particular types of the devices will be, for the most part, determined by the particular environment in which the invention is to be used. The details of such devices are well known in the art and no further explanation of their construction and mode of operation is necessary. 
     If the invention is used in the environment of an automobile as mentioned above, a horn switch 14 is provided which is connected to a horn fail safe circuit 88. The horn fail safe circuit 88 is, in turn, connected to the computer 64 and to the top link 26b. This circuit will connect the horn switch directly to the conductive link 26 in the event of a communication failure between the computers 40 and 64 or in the event of a power supply failure. 
     Data is transmitted over the conductive link 26 by interrupting the power provided by the link power control 50 for a short interval and inserting a data packet from the computer 40 through the link transmitter 42. This first data packet is then followed by a data packet generated by the computer 64 at the other end of the link 26. Power is then reapplied by the link power control 50 under the control of computer 40 and the process repeats indefinitely. The data packets which are transmitted and received are collected into message packets which are transmitted serially with each data byte being formed into a data packet for transmission between the two controllers. These message packets are transmitted continuously even when the message packet contains no data, in which case a null message is sent. In this manner the central controller continuously monitors the integrity of the link 26. This is to ensure that the link 26 will always be ready to send data when it is necessary, and also ensures that horn messages may be capable of being sent if necessary, as well as transforming clocking information to the remote substation, since the rate of data packet transmission equals the clock rate. 
     The power supply 48 is supplied with voltage from battery 46 and conditions this power to remove electrical noise and also provides fuse protection. The power supply circuit 48 provides filtered and fused battery voltage to the link power control 50 and regulated 5 volts to other circuitry. When power is first applied, the watchdog timer 58 applies a reset command for a short time to the computer 40 to force it to a known state. When the reset function is completed, the computer 40 pulses the watchdog timer 58 several times to be sure it will not reset the computer 40 again, and then proceeds to go to its initial operating state. When the computer 40 has finished checking its memory, setting the state of its output ports, and other housekeeping tasks, it turns on the link power control circuit 50 and then applies current limited power to the link 26a. 
     Next the computer 40 pulses the horn protection circuit 60 to enable the power applied to the link 26a to pass through the horn protection circuit 60 and on up the link 26 to the remote substation 16. The computer 40 then waits a short interval to allow time for the computer 64 in the remote substation 16 to stabilize due to the sudden application of power, and to allow time for the horn fail safe circuit 88 to remove the failsafe horn information from the link 26. Next, the computer 40 attempts to establish communication with the computer 64. If communication cannot be established within a prescribed interval, the computer 40 will deactivate the horn protection circuit 60 permitting the failsafe horn information to again control the horn operation, and if instructed, will also notify the accessory controller 54 of link failure. The process of alternately attempting to establish communication and then restoring the horn to its normal operation will continue indefinitely until communication is established. In addition, if communication is interrupted for a predetermined interval after being established, the process will again be invoked. In this way normal functioning of the horn is always guaranteed in the event of failure of communication between computer 40 and computer 64. 
     FIG. 5 shows in detail portions of the central controller 24 shown in FIG. 2. Link power control 50 receives a 12 volt DC supply from power supply 48 as powered by battery 46. This circuit is under the control of the PWR output of computer 40. PWR goes low whenever it is desired to apply power to the conductive link 26. This power is needed to supply the voltage necessary to run all of the components associated with remote substation 16 and, as will be explained below, the transmission of power is time-shared over the conductive link 26 with the transmission and reception of data. The link power control circuit 50 is a current limited source which supplies approximately 0.5 amps to the conductive link. When PWR goes low current flows through forward-biased diodes D11 and D12. The voltage drop across this diode pair is sensed across the base-emitter junction of Q11 and resistor R2 causing Q11 to conduct until the voltage drop across its base emitter junction and the voltage across R2 are equal to the voltage applied by D11 and D12. Due to the beta factor of Q11, the current in the base circuit and in D11 and D12 is much less than the current in the emitter of Q11. This causes the voltage drop across D11 and D12 to be significantly different than the drop across the base-emitter junction of Q11 due to heating. When PWR goes high, no current is able to flow through the base circuit of Q11 thus turning it off. D13 is used to dissipate the excess base charge of Q11 to decrease the turn off time of Q11. R3 is used to dissipate the excess power when base current flows in Q11 so that the power is not absorbed by the drivers (not shown) buffering the output of the computer 40. 
     The false sync prevention circuit 52 includes diode D1 and Resistor R5. When power is applied to the bottom link 26a, the voltage on the link is approximately 12 volts. This voltage back-biases D1 shutting off current flow from the 5 volt supply. When power is removed, the voltage on the conductive link drops toward zero. At approximately 4.3 volts, diode D1 begins to conduct current through R5 to the conductive link to keep it from going lower, thus providing a pull-up for the link transmitter 42 and preventing the link receiver 44 from interpreting the loss of power as a data signal. 
     The link transmitter 42 is controlled by signals from the computer labeled LTX which are placed on the link 26 through Q2. Resistor R4 provides base current to turn on Q2 which then provides a logic low signal to the conductive link 26 when it is desired to send logic zero data up the link. When LTX goes low under the control of the computer 40, the current through R4 is routed away from the base of Q2 thus turning it off. The computer 40 assures that PWR and LTX are never active at the same time, i.e., power and information are not supplied to the link 26 simultaneously. 
     Link receiver 44 comprises a comparator IC5 and associated resistors R7, R8, R9, R10, R11 and R12. Pin 1 of IC5 is an open collector and R8 acts as a pullup resistor to 5 volts. R10 and R11 form a voltage divider which together with hysteresis supplied by R9 set up the switching levels at which data present on the link 26 will be detected. The voltage differential between the two levels represents the noise margin for receiving data from the link. Resistors R7 and R12 provide the comparator with one-half the voltage present on the conductive link 26. As the voltage levels on the link are varied due to communication, a conditioned version of the signals are provided to the computer as LRX. In addition to assuring that the received signal must transition between defined levels, the link receiver 44 also isolates the computer 40 from the voltages present on the link 26 during power pulses. 
     The horn protection circuitry 60 and the horn control circuitry 62 are provided for applications of the invention in which a safety feature such as a horn or other signalling device must have access to the conductive link in the event of failure of either computer 40 or computer 64 which would otherwise control communication of this information along the link. The horn protection circuit 60 includes a relay K2 which is shown in FIG. 5 as being in an open position as would be the case if a fail safe mode of operation were needed. Ordinarily K2 is maintained in a closed position thus providing a path from the output of the link transmitter 42 and the link receiver 44 over the link 26. K2 is maintained in a normally closed position by pulses labeled LNKCLK which are pulses generated by the computer 40. Q3 provides the current path to turn on K2 and R36 stabilizes the action of Q3 with temperature. IC51 (pins 12, 13 and 14) is configured as a unity gain buffer. At initial turn on, C15 is discharged through R34 and the output of IC51 is 0 volts, thus turning off Q3 and K2. When the computer pulses LNKCLK, the pulse is sent through C14 to rectifiers D52 and D42. The positive edge of the pulse is sent through D52 to charge C15 and D42 then conducts on the negative edge. The positive pulse causes C15 to charge up, thus raising the output at pin 14 of IC51 to positive, turning on Q3, and pulling in K2. This connects the conductive link to the transmitter 42 and receiver 44 as well as to the link power control 50 and false sync prevention circuit 52 and removes the fail safe horn path to relay K1 in horn control circuit 62. As soon as C15 has charged and the positive pulse is removed R34 begins discharging C15; but as long as C15 is charged again before the voltage reaches a level that causes Q3 to turn off, the relay K2 will remain active. Should LNKCLK fail to pulse for a period of time, however, C15 will discharge and relay K2 will open, reconnecting the link to K1 for fail safe horn operation. 
     The horn control circuit 62 works similarly to the horn protection circuit 60. The output of relay K1 is connected to the horn 36 under normal operation with K2 closed. The horn is operated by a computer output labeled HORNCLK. When the horn switch 14 is closed, a signal message is sent by the remote substation 16 down the link 26 to the central controller 24 which in turn provides the HORNCLK pulses. The positive edge of each pulse is used to charge C7. The voltage on C7 is buffered by IC51 (pins 8, 9 and 10) and then used to turn on Q4 which activates the horn relay K1. Should HORNCLK fail to be pulsed, K1 opens turning off the horn. This prevents the horn from being inadvertently activated should either computer fail. During fail safe, relay K2 is open as described above, thus connecting relay K1 directly with the link 26 where it can be activated by closure of the horn switch 14. As long as the computer is in its normal operational mode, however, LNKCLK will periodically pulse, keeping K2 closed, and the horn will be operated only by the HORNCLK pulses from the computer 40. 
     Referring again to FIG. 3, when power is applied to the link 26, it appears at the input of the syncronous power switch 70 which it passes through when the amplitude exceeds approximately 8 volts. This charges energy storage element 76 which includes a relatively large storage capacitor. The capacitor in energy storage element 76 charges to approximately 12 volts and provides energy to the logic regulator 74 which then supplies +5 volts needed to power the remote substation components (connections to all such components are not shown). These components may include, for example, the data output devices 84 and the data entry devices 20. The logic regulator 74 supplies power to the reset circuit 75 and the false sync prevention circuit 78. The reset circuit issues a reset command to the computer 64 to force it to a known initial state. 
     At the completion of the reset command the computer 64 begins executing its stored program and sets the state of all its output lines. It then waits for a short time to allow circuits to stabilize and is then ready to receive a signal from the top link receiver 68 in the form of a start bit that would signal the beginning of a command or data. When power is removed from the link 26, synchronous power switch 70 becomes back-biased due to energy stored in the energy storage element 76 and turns off. At the same time, the false sync prevention circuit 78 supplies approximately 4 volts to the link 26 and prevents it from reaching zero voltage potential which would cause the top link receiver 68 to indicate a start bit. The power must be restored fairly quickly so that the energy storage element 76 does not become depleted. When the power is off, the central controller 24 forces the signal on the link 26 to 0 or a voltage near 0. A pulse approaching 0 volts causes the top link receiver 68 to indicate to the computer 64 that a start bit and clock edge have been received. The computer 64 then receives data from the receiver circuit 68. When a byte has been received, the computer 64 stores it in its internal memory, and after a short wait for a final stop bit, sends its own start bit to the top link transmitter 66 which then pulls the signal on link 26 to 0. After transmitting one byte of information, the computer 64 issues continuous stop bits to release control of the conductive link 26 to the central controller. The computer 64 then determines whether the received byte was the last byte of data in a complete message packet. Data bytes previously received from the computer 40 are then interpreted as a message packet, and the data extracted is sent to the data output devices 84 via the data output communication circuit 86 where the data is displayed to the operator or some other function is performed. The computer 64 then scans the data entry devices 20 through interface 82 for any new incoming data. New data is stored internally in the memory of the computer 64 and saved for processing later. Computer 64 then examines the state of the ambient light detector 77 and determines whether to adjust the level of the lighting for the display circuits of data output devices 84. Finally, the computer 64 examines its memory to see if any remaining message needs to be transmitted, and if not, examines the state of horn switch 14 and combines the present state of horn switch 14 with the data saved earlier from the data entry devices 20 to form a new message. Next, the computer 64 waits for a new start bit from the top link receiver 68 at which point the whole sequence starts over again. 
     Referring now to FIG. 4 the top link 26b is coupled to a horn fail safe circuit 88 which includes a relay K3. The other contact of relay K3 is connected to a pull-up network consisting of diode D7 and resistor R20. The zener diode D4 and resistor R43 ensures that whenever power pulses are present on the link these will not damage the computer 64 to which the line carrying the HORNSW signal is connected. HORNSW is the signal that results from pushing the horn switch 14 which is shown schematically as the input to K3. Normally K3 is closed, but if there is a loss of power, it will open, connecting the switch 14 to top link 26b. 
     The synchronous power switch 70 comprises SCR D3, capacitor C5, resistor R11, resistor R16 and diodes D5 and D6. When power pulses of approximately 12 volts are present on the top link 26b, diodes D5 and D6 conduct, turning on SCR D3 through C5 and R11. The power pulses are routed through the SCR to the energy storage element 76 and to logic regulator 74 which comprises voltage regulator IC6 and the display regulator 72 comprising voltage regulator IC7. The logic regulator 74 provides a 5-volt supply filtered by capacitor C3 for operating the computer 64 and its associated components, and the display regulator provides an 8-volt signal filtered by C4 for operating the data output devices 84. Levels of voltage on the top link 26b that are under 8 volts are not sufficient to conduct through the combination of D5 and D6, and SCR D3 remains off. Data pulses are on the order of 5 volts and do not pass through the SCR. In devices where the horn protection circuitry are not needed SCR D3 may be replaced by a simple diode and D5, D6, R16, R11 and C5 are omitted. In this environment all energy exceeding the voltage on the energy storage element 76 is supplied to the energy storage element 76. The top link 26b also includes a false sync prevention circuit 78 which is similar to that shown in FIG. 5 and consists of diode D2 coupled to the top link 26b through resistor R10. An energy storage element 76 comprises a large storage compacitor C2 which stores a charge provided by the power pulses gated through by SCR D3. 
     The link transmitter 66 comprises an open collector type of inverter IC4 coupled to the base of transistor Q1 which operates in substantially the same way as the link transmitter 42 of FIG. 5. Similarly, link receiver 68 comprises a comparator IC5 which operates in substantially the same manner as link receiver 44 in FIG. 5. 
     Communication between the central controller 24 and the remote substation 16 is established as shown in FIG. 7. Shortly after issuing the start bit, the computer 40 transmits a data packet serially to the link transmitter 42 and then returns the line to idle. After transmitting a data packet, the computer 40 waits for a predetermined time to receive a data packet from computer 64. Upon receiving the responding data packet, or if the alotted time has expired, the computer 40 turns on the link power control circuit 50 again in order to restore power to the remote substation 16. This cycle is repeated at preset intervals as determined by the central controller 24 and continues as long as normal operation is maintained. As data packets are received from the computer 64 they are examined for an indication that the message is a valid message. This could be implemented by requiring that a logic 1 bit be placed in the most significant bit position which would then signify the beginning of a message packet. The particular format used for transmitting messages and for determining their validity is not critical to the invention and those skilled in the art might well chose to use other formats and protocols depending upon the particular application. 
     As each message packet is received from the remote substation 16, it is processed to determine whether a valid message is received, and if valid, the data is saved in internal memory. The message is then examined for horn switch information and if the horn switch is indicated as active, the horn control circuit 62 is pulsed by the HORNCLK signal to activate the transducer 36. 
     In some instances, it may be desirable to monitor the power level in the energy storage element 76 by the use of energy level detection circuit 80. In these cases, one bit of information in a byte of data may indicate that the next transmission should be power rather than another data byte. Alternatively, the length of time that the link power control circuit 50 provides power over the link 26 could be lengthened. In either event, the average amount of energy per unit time will be increased to meet the needs of the energy storage element 76. A form of the energy level detection circuit 80 is shown in FIG. 8. The voltage on capacitor C2 of FIG. 4 comprises an input to a comparator amplifier IC100. The other input is a reference voltage level. The output at pin 7 of IC100 is normally low (POWERON). When this line goes high, it indicates that the voltage on capacitor C2 has dropped below an acceptable level and needs to be boosted. The computer 40 may then alter the length of the signal that it provides to link power control 50 to hold it on longer between the transmission of message packets. This will continue until voltage is restored to capacitor C2, at which time POWERON goes low and the computer 40 returns the signal PWR to its normal state. 
     Loss of communication between the central controller 24 and the remote substation 16 is a potential problem, and it is therefore necessary to provide a reset function to restart communication in the event of a temporary failure, and to ensure that the safety feature is connected to the conductive link 26 so that it can be available even in the event of a communications failure. 
     A watchdog timer 58 is shown in FIG. 6 which provides a reset pulse to computer 40 in the event that timing elements in the watchdog timer circuit 58 time out before being pulsed by the WATCHCLK pulse from computer 40 within a preset period of time. This timer operates in essentially the same manner as the timers described above in connection with FIG. 5 as the horn protection circuit 60 and the horn control 62. The computer 40 periodically provides a pulse which keeps capacitor C91 charged. In the absence of a WATCHCLK pulse C91 is allowed to discharge. If it discharges for too long, comparator IC5 goes high sending a reset pulse to the reset input of computer 40. 
     The computers 40 and 64 operate according to computer programs entitled &#34;bottom link&#34; and &#34;top link,&#34; respectively, which are reproduced in an appendix immediately preceeding the claims. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow. ##SPC1## ##SPC2## ##SPC3##