On Thu, 12 Aug 1999, Byron A Jeff wrote:

> >
> > How about CSMA/CD?
>
> It's not clear if the physical layer of RS-485 allows for multiple transmitter
s
> or what happens if multiple transmitters transmit different bits.

If you are doing half-duplex RS-485, you just tie the UART tx line
(inverted) to the RS-485 driver's R/W pin and tie the tx pin on the driver
to ground. That way, the driver will only transmit when it wants to send a
0. A collision is defined when one transmitter is sending a 0 and another
transmitter wants the line to be floating (i.e. it is sending a 1). There
is an appnote on this somewhere (National Semi i think). Collisions can be
detected in hardware with by: UART framing error, UART missing stop bit,
UART parity error or in SW with CRC errors in the frames.

> Essentially I'm doing the multiple access part because even with
> ethernet there is only a single transmitter on the channel at any
> point in time. This protocol handles channel selection on a separate

Well, there is only one transmitter sending error free data. When there
are two transmitters (which is possible in ethernet) a jam signal is
generated and both transmitters backoff.

> Any commentary on the UDP/SLIP part of it? The responses I've seen have been
> physical layer only which isn't a relavent as the protocols on top of that
> layer.

Only problem with UDP is that there is no guarentee that the data will get
there correctly so if your physical layer is not error free you have to
add a CRC field the data section of your UDP packet.

Slip/UDP is cool but in the end there just isn't enough processing
power/memory in a low end PIC to really make it worth while. You would
probably be better off using a custom protocol and then have one big PIC
convert from this custom protocol into Slip/UDP.

my $0.02

MD