Different ALDL data streams:
Early GM ECUs provide limited information about their operational status, and the status of the vehicle. This is provided at the leisurely rate of about 20 bytes per second (160 baud). Later ECUs provide more information at the faster rate of about 800 bytes per second (8192 baud), but they only output a fixed amount information. Setting the ECU to output this information requires a mode sense resistor across two of the ALDL connector's pins.
Still later ECUs, as well as transmitting data, could receive commands from diagnostic equipment and other in-vehicle computers. With these ECUs you can (if you know how) modify their internal operating parameters on the fly. These ECUs have a single bi-directional ALDL data stream, and require a special interface before commands can be sent, and data received.
8192 baud ALDL description:
The low level logical format of GM's 8192 baud ALDL data stream is a simple async data stream with 8 data bits, no parity bit, and 1 stop bit. This is the type of data stream that most UARTs (Universal Async Receiver Transmitter) can handle. The PC (IBM clone) has a UART that can be set close to 8192 baud (actually 8226.6 baud, using a divisor of 14 with the 115,200 Hz UART clock - giving a 0.42% fast clock, a negligible difference).
8192 baud Frame Format
Commands to the ECU are organised into groups of bytes called command frames.
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Originally Posted by g8hsv
