Class II Serial Data

The Class II serial data circuit allows the control modules to communicate with each other. The modules send a series of digital signals pulsed from approximately 7 volts to 0 volts. These signals are sent in variable pulse widths of one or 2 bits. A string of these bits creates a message that is sent in a prioritized data packet. This allows more than one module to send messages at the same time without overloading the serial data line. The speed, or baud rate, at which the control modules communicate depends on the message content. Large message content lowers the baud rate, while small message content increases the baud rate. The average baud rate is approximately 10.4 Kbps (10,400 bits per second). When the key is ON, each module sends a State Of Health (SOH) message to the other control modules using the Class II serial data line. This ensures that the modules are working properly. When the module stops communicating, a loss of the SOH message occurs. The control modules that should receive the message detect the loss and set a loss of communication DTC.