GM ALDL interface

[steve ravet]

> steve ravet <steve at imes.com> wrote:
> >The maximum byte length would be 5.75 msec.  At 8
> >data bits, one stop bit, you would need a baud rate of 1739.
> 
Ludis Langens wrote:
> I compute something different:  The length of the start bit and 8 data bits
> is 288/65517.1875 seconds (~4.4 mS).  So the baud rate for 8-N-1 should be
> 9 * 65517.1875 / 288 = 2047 baud.  (This assumes the timing used by my ECM.)

Well, you've gotten it to work and I haven't, but I still have to put my
part in. For an 8 bit RS-232 stream, there is a start bit, 8 data bits,
and a stop bit, for a total of 10 transmitted bits per byte.  The total
length of one ALDL bit is 5.75 msec:

    |---------6.25 msec-------|
    |                         |

----      --------------------
    |    |             |      |
     ------------------        ------

    |                  |
    |-----5.75 msec ---|

10 bits in 5.75 msec = 1739 bits/sec.

You could use a shorter length word which would lower the baud rate
some, but 8n1 is pretty common as far as comm programs go.


> 
> >...and a "zero" bit would
> >result in a bit [byte] with at least some ones in it (non zero).
> 
> The byte would be all ones.  The "zero" pulse is (exactly) 1/12 the "one"
> pulse.  This might actually cause problems with some UARTs because the pulse
> is slightly too short to be a start bit.  At 8-N-1, the start bit should be
> 1/9 of the long pulse.  Going to 8-E-1 would change the expected ratio to 1/10.

My spec makes it sound like the rising edge for a zero bit can fall
anywhere in a 1 msec window, which could result in some zeros...  But
maybe they are all as you measured.  The only other question I have is,
do you have to trigger the ECM into dumping the data or does it do it
all the time?  I don't have a scope, so I'm just going to hook it up and
see if I get anything.

> A frame format has been posted here before.  It turned out to be a 95% match
> for my case.  I've figured out the format for mine and another ECM.  One is a
> subset of the other, except for differences in some of the flag bits.

I've been through the archives but I can't find it.  Would you mind
posting it again?


--steve


--
Steve Ravet
International Meta Systems
http://www.imes.com
steve at imes.com