ALDL to Serial connector: Read this

[Daniel Burk]

I must put out a warning:  Many of the ECM's from the early 80's do NOT
output TTL levels, but a SES lamp voltage: aka 12 Volts!  My Trans Am 
does.  

The 8192 data rate seems to have been implemented on a very few 1985 
models, and on most of the 1986 models.  Most of the 160 baud systems 
have a 20 to 25 byte data stream, while the 8192 streams will contain as 
many as 47 bytes.

The modes are actuated by the resistance that you place across the pins A 
& B.

If you use a 10K ohm resistor, you get one of the modes. a 3.9K resistor 
triggers another mode, and 500 ohms or less triggers the "paperclip" mode 
that blinks the SES lamp.

Some ECMs support both the 160 baud and the 8192 baud data stream.