Old ALDL Synchronous format

[Gary W Harris]

     Hi all
     
     Someone a bit ago requested the information on the old GM ALDL 
     synchronous data format.  Here is what I know about it:
     
     ; The data frame consists of a start pattern of 9 ones, (the frame 
     ; sync pattern) followed by the actual data.  Each data word is 
     ; 9 bits, but the first bit is always a 0. Thus, the leading bit is 
     ; stripped away, leaving the 8-bit data byte. MSB is transmitted 
     first.  
     ; The total frame length is 19 data bytes (?).
     ;
     ;
     ; The following is a list of the bytes in order of reception:
     ; Note-- the address is controlled by table @ 0d4c5h for type 4E ECU
     ; an interesting aside--can change RAM/ROM location that is 
     ; monitored by altering address in EPROM table
     ;
     ; A/D conversion values based on 256 counts = 5.000 volts (not 255 
     counts!)
     ; Thus, 1 bit = 19.53 mv.
     ;
     ;  ID_HIGH     high byte of eprom ID#, location 0d002h
     ;  ID_LOW      low byte of eprom ID#, location 0d003h
     ;  IAC         current idle air count
     ;  COOLENT     coolent temp, deg C= (0.75 x value) - 40
     ;  MPH         miles per hour
     ;  MAP         manifold air pressure KPA = (0.3125 x value) + 20
     ;  RPM         RPM = 25 x value
     ;  TPS         throttle position, % = (100 x value) / 255
     ;  INT         integrator value
     ;  O2          o2 sensor value, mv = 4.50 x value
     ;  ERR1        error codes byte 1
     ;  ERR2        error codes byte 2
     ;  ERR3        error codes byte 3
     ;  AFSTAT      air/fuel status byte
     ;  BATT        battery voltage, Volts = value x 0.195
     ;  IOSTAT      descrete IO status byte
     ;  KNOCK       knock counter (rolls over)
     ;  BLM         block learn mode value
     ;  ALDL        aldl counter (rolling--frame cntr?)     
     ;
     ;  
     ;
     ;
     ; Use 2 EPA channels to perform the serial input function.  
     ; EPA0 is set-up to detect a falling edge on PORT 1.0, then 
     ; EPA1 is used to sample the bit value in the middle of the 
     ; the BAUD period.
     ;
     ;
     ;               6.250 mS (=1/160 baud)
     ;         |<------------------------------->|
     ;         |                                 |
     ;
     ;---------+     +\\\\\\\\\\\\\\\\\\\\+------+     +\\\\\\
     ;         |     | data here (1 or 0) |      |     | next data bit
     ;         +-----+\\\\\\\\\\\\\\\\\\\\+      +-----+\\\\\\
     ;
     ;         |     |         |          |<--->|<---1.65 mS high always
     ;         |     |         | 
     ;         |     |         |
     ;         |     |         +--sample bit here, t=2.400 ms
     ;         |<--->|            from falling edge bit start
     ;            |
     ;            +--200 uS start time (low always)
     ;
     ;
     
     
     Cheers,
     
     Gary