Hi !

John Esposito wrote:
>
>
> For the next phase of my PIC project (home security), I want to have remote
> keypads communicating (only) with the master control unit.  I originally
> wanted to use RS232, but was under the impression that RS232 is difficult
> to deal with when using long runs (i.e. through a house) and multiple
> interface nodes.

For both RS485 and RS232 you can increase the maximum distance by
decreasing
the speed (boudt rate). I expect for interfacing a keyboard or card
reader
you wouldn't need more then 1200 bps.

For future expandability of the "network", I am leaning
> toward RS485, using a Maxim interface chip.  However, I will not run it as
> a multi-drop configuration, but as a star-type configuration, with the
> keypads running (throughout the house) directly to the master.

If you want to do the termination right, you can use at your control
unit
one transmitter and receiver driver for each line and make the
connection of
all signals on the TTL side of the drivers. So physically every line
will have
a driver circuit (and a terminator for RS485) on each end.

It costs more, but imho it makes sense for a security system. If someone
attacks one of the lines and blows the driver circuit away with a high
voltage applaied to the cable you can manage it to keep the other
keypads
working.

>
> My question is:  is my impression of RS232 unfounded?  If so, why?  Has
> anyone used RS485 in the way I want to use it?
>

Haven't tried it myself. There are some ideas :

- Terminate only one side of the line (at the transmitter). Then a
reflected
wave travels back from the end to the transmitter and is absorbed by the
terminator and doesn't affects the next transmission.

- You can terminate only the AC part of the signal by putting a C in
series
with the terminator. So the voltage wouldn't drop if you connect
parallel
more terminated cables.

- If not terminating at all you can use diodes to clamp the positive and
negative overshoot of the reflected wave. Such diodes are build in most
ICs.


If cost is a problem you can try it first by reducing the speed and by
using
a check sum in the transmission protocol to make it immune to errors.
(That's
the way the one-wire bus by Dallas works: no termination, slow speed and
a CRC check.)


Just my 2 cents ...
St.