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PICList Thread
'Error rates for SCI pic16c74,'
1996\07\01@034327 by NEIL GANDLER

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I plan on using the SCI modules of two PIC16c74 for full duplex
asychronous communication. I plan to run them at 1200 baud with a 4mhz clock.
There are no specs for error rates in the PIC manuals. I plan
on linking them with twisted pair wire, maximimum 15ft in length, in an
automobile. I was wondering if the 9th bit parity option is adequate for
error detection. Just to be safe, I designed my software to send a 32 bit
packet and then resend an identical
copy. If they both matched, the receiver will assume all bits are correct.
That would achieve excellent error detection yet eat up quite a bit of
transmission time, but still could be practical.
The only reason I want to keep transmission speed as low as
1200 baud and so concerned about error detection, is that I may want
to link these these PICs using Ming RF digital modules, which add
considerable to error rates and allow maximum speeds of 1200 baud. I
am much more conerned with detecting errors than correcting or eliminating
them. I would also appreciate any programming and implementation advice,
from people who have used the SCI successfully. I can't believe this
thing looks so easy to use, I wish I read about it earlier.

               Neil Gandler

1996\07\01@044458 by Eric Smith

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NEIL GANDLER <spam_OUTV064MB9KTakeThisOuTspamUBVMS.CC.BUFFALO.EDU> writes:

> I plan on linking them with twisted pair wire, maximimum 15ft in length,
> in an automobile.

Cars are very electrically noisy, so I'd strongly recommend using RS-422
or RS-485 balanced pair wiring.  Especially if any of this stuff is in the
engine compartment.

Or better yet, use optical digital audio interconnect.  It uses cheap plastic
fiber and there are inexpensive transmitter and receiver modules with TTL or
CMOS level I/O.  Toshiba sells them under the "Toslink" trade name, and there
are other brands.

Also, if you are going to power this stuff from the car's electrical system,
you need really good power supply filtering.

National Semiconductor makes some voltage regulators specifically for
automotive environments, which can withstand subtantial input overvoltage
conditions as well as reverse voltage.  I don't have part numbers handy.

Cheers,
Eric

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