(Fwd) Mail Delivery Failure.

steve ravet steve at sun4c409.imes.com
Fri Apr 11 18:10:56 GMT 1997


I looked thru the archives for the post I made, here it is.  This
information comes from GM training materials (I believe).  Anyway,
enjoy:


The 160 baud stream is different.  It's not really serial, in that
each data bit is preceeded by a falling edge.  The line is driven to
the inverted data value 1.5-2.3 msec after the falling edge.  After
this sample interval goes by, the line goes back high, and 6.25 msec
later another falling edge indicates that another bit is coming.  This
won't connect to any serial port, but it wouldn't take much to have a
little programmable logic convert these into bytes and feed them to a
UART.  Here's a diagram:


______     _____________
      |   |        |    |
      |___|________|    |
     T0   T1  T2   T3   T4

T0=falling edge that indicates the start of a bit (not byte).
T1=rising edge, if the bit is a zero.  If the bit is a one, the line
stays low T2=sample period T3=rising edge to prepare for next bit
T4=start of next bit

T0-T1 = 1.5 ms minimum.
T0-T3 = 2.3 ms maximum.
T3-T4 = .5 ms minimum.

To extract bits, wait for a falling edge, then sample from 1.5-2.3ms
later.  This waveform is repeated for each bit.

Data is sent as nine bit bytes, with the first bit being a start bit.
The remaining 8 bits are sent MSB first.  The very first byte is a
sync character.  It has a "1" start bit, and all "1" data bits, ie 9
ones in a row.  The data bits then are sent, each with a start bit of
zero.  When all the data has been sent, the whole thing starts over
with the sync character followed by the data bytes.  I don't have any
information regarding what the bytes actually mean.

--steve



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