[Gmecm] j1850 and/or 68hc58 experience?

[Steve Ravet]

Some of you may recall that I'm working on a cable that allows you to
connect a PC to a GM OBD2 computer via USB.  I've made some progress,
I'm able to send the "shut up" message to the PCM to get it to stop
broadcasting messages, but I can't read data back from the DLC.  I'm
using the hc58 in serial mode and have some questions about that:  Do
the status bytes you get back while transmitting a message mean
anything?  I can't make sense out of what I get back from the DLC while
transmitting a message.  And after transmitting a message, for example
requesting the VIN, I don't get any response.  On the scope I can see
the broadcast messages the PCM sends out but I never see any bytes in
the receive FIFO.  Here's a trace of the VIN request:


       data    command
       0x6c    0x14
       0x10    0x4
       0xf0    0x4
       0x3c    0x4

       status   receive
       0x00,    0x00  (receive and send FIFOs empty)
       0x7b,    0x00  (receive FIFO full, end of frame not seen, link
idle, link fault, send FIFO full)
       0x01,    0x00  (receive FIFO empty, send FIFO has data)
       0x18,    0x12  (receive FIFO empty, link idle, link fault)

       data    command
       0x1    0xc
       0x0    0x0
       0x0    0x0
       0x0    0x0

       status   receive 
       0x00,    0x00
       0x7b,    0x00
       0x01,    0x00
       0x18,    0x12

The transmit message is 6c 10 f0 3c 01, which requests the VIN.  The
status bytes don't make any sense to me, two of the status bytes report
a link fault, which means the J1850 bus is shorted to ground.  You can't
clear this status bit except by resetting the DLC which I'm not doing.
Plus the FIFOs going from empty to full and back to empty doesn't make
sense, the FIFO is 12 bytes deep.

--steve

-------------------
Steve Ravet
ARM
steve.ravet at arm.com

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