>I am currently working on a network of PIC controlled units for
>minitoring machines in a manufacturing plant.  The units will connect to
>a 485 bus and will be interogated by a PC (master/slave setup).  I am a
>bit unsure about the practical aspects of such a bus in an industrial
>environment since this is my first attempt at it.  I would appreciate any
>comments from people who have done this before:
>
I have been thinking of putting uo a web page on this with source
and stuff that we use. I will post something to the list.

>I was thinking on routing the bus through each slave unit.  You would
>have a female plug going in and a male plug going out.  If the unit needs
>to be removed, then the two parts of the bus can simply be plugged into
>each other.  The alternative is to use T-style taps from the bus, but
>I am not sure how to make the T connections easily and reliably.  Also,
>is DB9 plugs/sockets acceptable in the industrial environment?
>

No you really don't need to route in/out. DB9's may be ok
but it really depends on the environment. I would not use them
in a wet area for instance. We use a three way phoenix screw terminal
depluggable, with the in/out just paralled on the connector.

>I have read in an 485 application note, that it is advisable to tie
>the grounds of all your nodes in a 485 system together to limit common
>mode problems.  Is this usually done?  Can I use a screened two wire
>cable and use the screen to tie the grounds?
>
Yes, grounds can vary quite a bit and in my experience grounding in
factory environments is generally not good. The common mode range of
the rs485 receiver is the problem. Tie the grounds together.
Smallish networks with good grounding probably do not need it.

>How critical is the 32-nodes-per-bus limit?  I am planning on using
>repeaters, anyway.

We use an rs485 repeater, but really it is more to allow us to use
different topologies, some more recent rs485 chips have much lower
loading and can comfortably run up to 256. Personally I would not do it.
On the other hand I know of systems where 100+ devices hang off the
single network, apparently without problems.

>
>What kind of protection is advisable at the input of each slave?  I was
>thinking about back to back zeners (say 6.8V) from A and B to ground to
>limit the voltage on each line to +-(6.8+0.7)V.  Is a zener good enough
>or should one use a Tranzorb or MOV?
>
We use MOV's although if you want lightening protection a purpose built
protection circuit will be more reliable in the long run usually based
on gas discharge they can really take a hit and keep working. First
question is to ask what are you protecting your system against!. I have
heard of mains power being accidentally connected to rs485 networks!

Very expensice mistake..

>I am communicating at 1200Baud.  Is termination really necessary?
>
Always terminate, at the ends of the network.




Ray Gardiner (DSP Systems) ray@dsp-systems.com http://www.dsp-systems.com
private email to:- ray@netspace.net.au