Sending
| Sending |
A control module sends a message by transmitting ones and zeros in a determined order. The control module sends the message only if no other control module is transmitting. What the various combinations of ones and zeros mean is determined in advance so that all control modules interpret the information identically. The message contains a description of what is sent and its value, e.g. ”Coolant temperature 100°C” or ”A/C On”.
The two bus leads are supplied with about 2.5 V from each control module. This is accomplished by means of three resistors connected in series between 5 V and ground. The middle resistor ensures that the current on bus+ is slightly lower than that on bus- when no messages are being sent.
When both leads are supplied with about 2.5 V for a certain period of time (2 millionths of a second on the P bus), a one will be transmitted. To transmit a zero, the control module pulls bus+ almost to 5 V and bus- almost to ground for an equal period of time.
Note that a long sequence of ones is the same as no message being transmitted.
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