Hi Tom,
I understand. It's a implementation of CSMA/CD like in ethernet
applications.
But
 - how do you make a back off on bit corruptions ? Do you check the
transmitted bit against the received bit on the TTL side with a XOR gate and
then generates an interrupt ?
 - how do you defined the jam signal ? Sending a break condition, sending a
number of 0's or what else ?

In the past time, there was a discussion about collisions in a RS485 network
(was this in this thread?).
The result was, that in this case the differential voltage on the RS485
lines is not defined. This is the same, what i have seen in my application.
I have done a CSMA/CA implementation on RS485 (i'm currently porting this on
a half-duplex FSK-Modem, therefor only CSMA/CA). All sender are synchronous
to each other. Each packet has a pending CRC. When more than one node are
sending, a) the packet was corrupted, b) a high or low was dominant, c)
simply receiving garbage, and d) the CRC was ok, but the payload not. The
latter case i didn't expected, but the possibility for it was very very low.
Additional hint: I have used a LT1785 with external pull-up/down resistors
for True-Failsafe. On additional test adapters, i've used a SN75176B. On
collision, the TI chip has given me a complete different data packet as the
nodes with the LT chip has seen !!!

Frank
---------------------------
GSP Sprachtechnologie GmbH
Frank Wollenberg
HW-Entwicklung
Tel.:   +49 (0)30 769929-78
Fax:    +49 (0)30 769929-12
eMail:  f.wollenberg@gsp-berlin.de


--
GSP Sprachtechnologie GmbH
Teltowkanalstr.1, D-12247 Berlin
Tel.:  +49 (0)30 769929-0
Fax:   +49 (0)30 769929-12
eMail: Info@gsp-berlin.de
Web:   http://www.gsp-berlin.de

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body