I wouldn't recommend RS232 for long runs if it can be avoided and it
is definitely outside the formal specification

BUT

at 300 baud it will run with essentially 100% reliability over very
large distances on wet string (well, maybe the water needs to be
salty :-)).
Think about it - you have a nominal +/- volt swing - say +/- 5 in
practice. At 300 baud you have 3ms per bit. With 1 mile/1600 metres
of wire you probably have say 500 ohms max resistance and 100 nF
capacitance. Time constant of this is about 50 microsecond or about
1/60th of a bit time. Faster baud rates will give narrower margins
but I'd expect reliable operation around a house at baud rates
significantly above 300 baud. That said, it's outside the spec - look
at the results on a 'scope and decide for yourself.

Summary: For around a house and SLOW data rates it will work. If you
can use differential drivers (RS422) then you'll get better
reliability.

(Being picky AFAIR RS422 is mono-drop and RS485 has the spec
extensions to cover multi-drop)


regards


        Russell McMahon

From: John Esposito <    >
>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 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.
>
>My question is:  is my impression of RS232 unfounded?  If so, why?
Has
>anyone used RS485 in the way I want to use it?  If so, did you use a
>terminating resistor as each node to minimize reflections?  Any
other
>information you could provide would be very helpful