> I want to use a 16F76 as a master for a burglar/fire home alarm system.  I
want
> to have maybe 4 16F628's around the house for remote access.  Using the
USART
> functionality, the 4 remote transmitters would be electrically be in
parallel to
> the 16F76's receiver input.  The data sheet does not state the 16F628
USART/TX
> (RB2 - pin 8) as open drain.  Will paralleling them be a problem?

Yes.  You should check out RS-485, which is basically what you are trying to
do except that the electrical details have been worked out.  There are
driver chips available the pretty much "convert" between the PIC TX and RX
lines and RS-485 given an additional direction control input.

> I don't believe there is a concern in the opposite direction, that is 4
16F628
> RX inputs electrically in parallel from the 16F76 TX.

Correct, but you should consider issues like noise immunity, protection from
inductively or capacitively coupled spikes, and line termination.

> I realize there is the RA4 pin on both IC's that are open drain, but
wanted to
> use the USART function.

You should be able to do what you want to do with RS-485 although you will
have to figure out the low level protocol details yourself.  Since this is a
single-master system, you could use two RS-485 busses.  One for sending by
the master and the other for sending by all the slaves.  You could also do
this with a single RS-485 bus, but have to get a little more fancy in the
protocol so that everyone on the bus agrees who is talking.

Another option is CAN.  Some 18 series PICs come with CAN built in.  CAN
takes care of the lowest protocol levels directly in the hardware.


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