Keith Dowsett wrote:
>
> Jeff Cesnik wrote:
> >
> > Does anyone have an example schematic (or know where to find one) using
> > two PIC16F84's communicating over an RS-422/485 link?  I've done RS-232,
> > but have absolutely no experience with balanced line communications.
> > Can someone recommend a good line driver/receiver suitable for
> > interfacing with the PIC?
> >
> > -Thanks
>
> Hi,
>
>    If you've got the code for RS-232 you should have very little trouble
> implementing RS-485 or RS-422.
>
> I've not used RS-422, so I'll restrict my comments to RS-485. I used the
> MAX485 but there are a whole range or transcievers which fit the same
> package. 8-pin PDIP and pretty cheap.
>
> The only thing you have to remember it that you need to include
> terminators at either end of the network (if you only have two nodes
> it's one each). You will probably also want to add a pair of resistors
> to bias the lines so that noise doesn't trigger the receive routines. I
> found a neat summary of this on http://www.bb-elec.com/techlibr.html
>
> I went for a two wire interface, data, and transmit/receive. The
> bit-banged 9600 baud comms.
>
> To send, a PIC checks that the line is inactive, then enables the
> transmit and sends it's serial data. By using PB0 as the data pin I was
> able to use the interrupt to trigger the receive routine. The routines
> used are pretty much the same as for RS-232.
>
> Hope this is some help,
>
> Keith.

There is an article, by Jan Axelson, with exactly what you're looking
for (and more) in the July/August 1995 issue of MicroComputer Journal.