The 160 baud rate is due to the ECM operating with 6.25ms minor loop interrupts. It is ok for slow data speeds but puts too much load on the microprocessor for high baud rates. Bit banging is a method to transmit serial data using only software rather than a hardware UART. The ECM uses a ‘bit-banging’ technique on the CEL (Check Engine Light) wire to flash data out bit by bit. It operates at 160 baud and is a uni-directional (read only) data link. VN/VP – This was the first implementation, was also the most basic and is the only implementation that uses ALDL for purely diagnostic purposes.
There are a few different implementations of ALDL… It is used for a variety of things from, as the name suggests, initial system testing when the vehicle is manufactured to vehicle module communications to diagnostics at the dealer/home to real-time tuning. What It is and What Can It Do: ALDL stands for Assembly Line Diagnostic Link.
About: This information will be updated from time to time and serves as a basic introduction to GM ALDL, specifically aimed at GM Holden Commodores.
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