Sniffing RS485
Mike
I have a RS485 bus that I want to sniff, with the view to understanding
the commications protocol being used and adding my own device to the
bus.
Does anyone know anything about doing this? One would assume one should
get a converter to convert it into something that can be fed into a
computer and then watched? What's the best thing to convert it to and
what software should one use? I've read it's possbible to convert to
RS232 and watch it with a terminal emaulator but only if it's ASCII?
Any advice would be greatly appreciated.
Regards,
Mike.
Mike Harrison
>Folks,
>
>I have a RS485 bus that I want to sniff, with the view to understanding
>the commications protocol being used and adding my own device to the
>bus.
>
>Does anyone know anything about doing this? One would assume one should
>get a converter to convert it into something that can be fed into a
>computer and then watched? What's the best thing to convert it to and
>what software should one use? I've read it's possbible to convert to
>RS232 and watch it with a terminal emaulator but only if it's ASCII?
There are plenty of serial port monitors out there that can do binary or ASCIII
>Any advice would be greatly appreciated.
>
>Regards,
>Mike.
The hardware is trivial - it;s just a case of level-shifting - you may also be able to get away with
connecting D- direct to an RS232 port, or D+ to a TTL-level serial interface like an FTDI cable.
The main problem with 485 is that as a multimaster bus, all data appears on the same lines, so it
can be hard to tell who's talking at any time.
The best way to avoid this is to open up the kit you're looking at and monitor the TX data before it
hits the transciever - that way you can know for sure which device is transmitting.
If that's not an option, and you have a scope, you can add series resistors to one device, so any
data from that device appears at a lower level on the scope so you can see who is talking when.
A good RS232 monitor that does accurate timestampting may also help seperate out packets to help
distinguish devices. Note this will need to use real serial ports not USB ones, as USB packetizing
will obscure any timing information.