[Gmecm] j1850 and/or 68hc58 experience?

[Darren Freed]

Steve,

The other thing you may want to consider is connecting an ELM-based device
on the Class 2 bus so that you can see what you really _should_ be expecting
to see.  I would think that would take some of the uncertainty out of it.

FWIW, the receive msg doesn't make any sense to me either.  You really
should see a response to the 'shut up' command (mode $28).  That mode #3c
request (VIN) has worked for me, using the an AVT device.

Darren


On 5/4/07, Steve Ravet <Steve.Ravet at arm.com> wrote:
>
> 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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