Hi Niki,

I havent actually tried this solution but I have considered it.....
Run your cables from a central point as you are proposing, but run CAT5
or similar and wire as follows....

Main Controller -----Blue pair in cable to 1st node then connect the Orange pair
to the blue at node 1

This sends the signal from Master to node 1 and ends up on the Orange pair
of Node 1 cable back at the central point.

Join the Orange pair of Node 1 to the Blue Pair of Node2
At node 2 join Blue and Orange

Join the Orange pair of Node 2 to the Blue Pair of Node3
At node 3 join Blue and Orange

etc...

At the final node terminate the Blue pair with a terminating resistor and also a
t
the Master.

The net result would be an effective continuous network without any taps TWICE t
he
length of your total cable runs but with only single cables to each location.
As mentioned earlier I HAVE NOT ACTUALLY TRIED THIS but I see no reason
why it shouldnt work.

Of course others may know better :-))

Best Regards

Keith

http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation

----------
From:   N Steenkamp[SMTP:STEENKMP@FIRGA.SUN.AC.ZA]
Sent:   18 September 1998 22:14
To:     PICLIST@MITVMA.MIT.EDU
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