>
> (2)  Somebody on the list  recently indicated that it was  easy to convert
> from RS232 to RS485, I have no problem at the PIC level but without
> purchasing an expensive RS485 Card for my PC which is  intended  to be the
> master controller in a multi-drop system I am having difficulty in enabling
> the RS485 driver from a standard PC RS232 port while transmitting  and then
> disabling the driver and still be able to listen  for a reply on the PC port.
>
I agree with Andy's recommendation to take a look at the
B&B Electronics web site: www.bb-elec.com
They sell converters (~$50) and also have good technical
info online.

One way to convert RS-232 to RS-485 is to use the RS-232
RTS line to switch the RS-485 transceiver from transmit
to receive.  My setup is this:

RS-232 from my PC <-> ICL232 (+ 74HC14) <-> DS3696 <-> RS-485

The ICL232 (Harris) converts from RS-232 levels to TTL.
(Many people prefer a MAX232 instead.)  Along with the
RX & TX lines, the RTS line from the PC is also converted
to TTL.  Since the ICL232 inverts singnals, I have to use
a 74HC14 to reinvert the signal so it can drive the TX/RX
enable pin of the DS3696 (National).

That's only the hardware part of the story.  You must
control the RTS line correctly on the PC (if you roll
your own converter as described above).  All the
terminal programs I tried did not unassert RTS after
transmitting.  What you want is for the RTS line to be
asserted immediately before sending a message, and
unasserted immediately after sending a message in order
to switch the RS-485 transceiver from transmit to
receive so you can get a reply.  You will most likely
have to write code yourself to do this.  You can do it
in assembly, in DOS using inp & outp calls (communicating
directly with the UART), or in Windows95.  Windows95
claims to have a mode which does exactly what you need,
but it doesn't work.  However, there are calls to set
and reset the RTS line explicitly which is what I used.

Keith

Keith D. Kotay
Ph.D. Candidate
Dartmouth College
kotay@cs.dartmouth.edu
http://www.cs.dartmouth.edu/~kotay