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Tuesday, August 21, 2018
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Position Of RS485 Modbus Master
Placing a Modbus Master inbetween modbus slaves not at beginning or end of a network

Dear experts,

In my project, I am establishing Rs485 Modbus Rtu Network. My problem that the master located in between of slaves. So, Can I put the Modbus Master in between of different Modbus slaves not in the start or end of network?

Thanks.

It doesn't matter where the master is located as long as both end of the network are properly terminated.

Ideally, the master is at one end of the cable and all the slaves run along the cable from there, with the last slave being at the far end of the cable. That gives you the best transmission line behaviour.

However it depends on how long the cable is. The idea is that any reflections die down within the risetime of the signal, otherwise you are relying on the termination to work.

We make various RS422/485 interface converters and we use reduced slew rate chips which work up to 115k baud and which don't need terminators (or any special wiring topology) for up to about 300 metres.

I hope this helps. It may well be that your cable is really short. But if it is say 1000m+ then you need to be more careful.