I have no practical experience of this, but ....
.
At 19200 bps a wavelength is C/19200/K
where C is 3 x 10^8metres/second and K is a velocity modifier.
For K = say 3,  a wavelength is about 5000 metres.
I'd say your  eg 30 meter stubs are going to be fairly harmless.
.
But then, as Carl Sagan was won't to say, I may be wrong.


        Russell.
-----Original Message-----
From: N Steenkamp <STEENKMP@FIRGA.SUN.AC.ZA>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Saturday, September 19, 1998 7:16 AM
Subject: Reflections in RS485 PIC Network?


>Hi,
>
>I have designed a network of PIC driven units that will be installed
in
>an injection moulding factory.  The network is a muti drop RS485
with a
>simple master-slave protocol.  The whole thing works beautifully,
but I'm
>a bit concerned about the effects of the network wiring when it is
>installed in the factory.
>
>I am planning on using an Ethernet-like structure where each unit
taps
>from a center connection.  My baud rate is 19200 and the taps can be
>quite long 10's of meters.  What would be the effect of all these
taps
>(since each tap is a discontinuity in impedance)?  Would I have
problems
>with reflections?  Also, I cannot put a termination resistor at each
>slave since the DC load would be too high.  Would AC termination be
a
>good idea to help minimize reflections from each slave back into the
>network?
>
>The alternative is to snake the central conductor to each node, thus
>elliminating the long tap wires.  This, though, makes the wiring
more
>difficult and more expensive.
>
>Lastly, I have 6V8 bidir-tranzorbs from each of the two differential
>lines to ground in each slave and make use of a shielded twisted
pair
>cable.  Is this sufficient (in terms of interference and transient
>protection) for such a network?
>
>I would appreciate any suggestions!
>Thanx
>Niki
>