PLC4 Dummies

Oct. 1, 2000
FOR many of us, the "black box" of vehicle electronics is a Pandora's box - one we have no idea what we will find when we open it.Even though PLC4TRUCKS

FOR many of us, the "black box" of vehicle electronics is a Pandora's box - one we have no idea what we will find when we open it.

Even though PLC4TRUCKS has been written about repeatedly, some of the mystery may still remain. But in just a few months, it will be standard on every truck trailer manufactured; so a working knowledge of the system may not be a bad idea.

Much like a telephone, PLC4TRUCKS uses a wire to connect the sender of the message with the receiver of the message. In this case, the wire is the auxiliary circuit (the "blue wire") that links the tractor and trailer via the standard J-560 tractor-trailer electrical connector. And like a telephone, both the sender and receiver must have a "handset" that sends the messages along the wire and receives incoming messages.

We can identify voices on the telephone because they have a tone and pitch that we know. The same holds true for PLC4TRUCKS. The system uses a computer chip that sends a signal or "chirp" that can be recognized or understood. The signal travels from the computer chip that is included in the ABS controller. When the receiver on the tractor identifies the signal, it responds. In the case of PLC4TRUCKS, the response is one of only two things - turn the light on or turn it off.

But that is just the beginning. ABS and other manufacturers have developed systems that can control other devices. The number of these devices is limited to the number of inputs and outputs (places to plug in wires) on the controller. Just as a single radio can play multiple radio stations, a single controller can identify the frequency of incoming chirps. When it does, it responds as it has been programmed to do - monitoring brake lining wear, turning on a dome light in a trailer, letting the driver know a door is open, or a variety of other functions.

Each device takes turns communicating. The communications protocol (the "language" that the computer chips speak) includes a signal that detects when the communication line is idle. If the device detects a busy signal, it waits to communicate until the line is available. Engineers serving on the Society of Automotive Engineers Powerline Carrier Communication Task Force have addressed the issue of conflicts between devices. Different devices are assigned priorities. If a lower-priority device tries to use the line at the same time as a higher priority device, the lower priority device must wait until the line is no longer busy.

PLC4TRUCKS will make a variety of new trailer options possible.

"There are all kinds of possibilities," says Jim Hofstetter, vice-president of engineering for Great Dane Trailers. "A lot of them have real potential, especially those related to brakes, suspensions, and refrigeration units. Of course, each will have an added cost associated with them. The trailer customer will have to weigh the value and the cost in deciding which of these options to buy."

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