Does anyone know what PICs do with a false start bit?  Here's the
background:  I am designing an RS485 network (single master) that will run
at 9600baud.  All devices on the network, including the master, are 17C756
based microcontroller designs with the exception of nodes I will use for
both power isolation and 485 bus extension.  Since these nodes include
isolated rs485 transceivers that require DE (driver enable), I am planning
on including one-shots in the nodes to enable their transceivers for a
period of time equal to 1.5 characters at 9600baud and I will allow these
one shots to retrigger while activated to handle data longer than one
character long.  As I read the 17C756 data sheet, I think I can put a lo on
the bus for less than 7(16xBRG clock) and thus enable the drivers without
causing all the slaves to think data is coming since the start bit is only
sampled at the 7th 9th and 11th bits by the slaves.  Does anyone see a
problem with this approach?  Thanks!

By the way, I plan to put the lo pulse out there by changing the baud rate
on the master's USART to 250kbaud (36us per 9 bits) and writing 0x00 this
should produce a lo for 36us which is less than the 7th bit time at 45.6us,
I will then change the baud back to 9600 and tx data from the master.  A
similar method must be used on reply data from the slaves back to the
master.

+++++++++++++++++++++++++++++++++++++++
++      Dan Tye
++      Chief Engineer
++      Tel-Tron Technologies Corporation
++      220 Fentress Boulevard
++      Daytona Beach, FL 32114
++      Phone: 904-255-1921    Fax: 904-258-3782
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