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In RS485 communication, what is the role of termination resisotor.

Is it mandatory?

I have been working on a project with MODBUS/TCP over RS485 where upto 32 slaves are there in some links, out of 300 links am testing. One of a problem i came across is, in most of the cases when there is a termination resistor, there is a problem in establishing a communication with the slaves. But as soon as i remove this resistor communication is back.

Now if it is mandatory, then can anyone help me find a solution, what i can do to resolve this termination problem. I have tried google and all answers but not getting a suitable answer.

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    \$\begingroup\$ What value resistor ? Where is it placed ? How are the devices wired ? The information is not sufficient (to address the second part of the body of your question). \$\endgroup\$
    – efox29
    Commented Jun 21, 2016 at 3:12
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    \$\begingroup\$ Also - see electronics.stackexchange.com/questions/32135/… \$\endgroup\$
    – efox29
    Commented Jun 21, 2016 at 3:14
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    \$\begingroup\$ Also, you could have googled up "rs485 no termination resistor". I'm just saying that you could have done alot more than immediately come to this site looking for someone to give you the answer. \$\endgroup\$
    – efox29
    Commented Jun 21, 2016 at 3:14
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    \$\begingroup\$ Modbus TCP/IP over RS485, haven't seen it. It's Modbus RTU/ASCII on RS485 hardware layer, and it's Modbus TCP/IP on ethernet hardware layer. Between those two different networks, there is a gateway or converter. \$\endgroup\$
    – Marko Buršič
    Commented Jun 21, 2016 at 7:58

2 Answers 2

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Some rules for RS485:

First, the network needs to be one single string of nodes - NO branches and short stubs. The allowable length of stubs is related to your data-rate, but to be safe, you should strive to eliminate them.

Second, now that your link is one long interconnect, you need a termination resistor at each end. The value of the resistor must match the characteristic impedance of the cable - between 100 and 120 ohms for your typical communication-grade twisted pair.

If you have more than two resistors, then the drivers may not be able to drive the cable, so intermediate nodes MUST NOT have terminators installed.

There should be no more than 32 "standard" nodes hung on the bus. Many RS485 transceivers are rated for "1/4 load" allowing 256 nodes.

The purpose of the resistors (and why they must match the cable) is to prevent reflections off of the end of the cable. The longer the network, and/or the higher the data-rate, the more critical the terminators become.

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  • \$\begingroup\$ +1 for stressing that "intermediate nodes MUST NOT have terminators installed". If all slaves had their own terminations the net differential signal on the bus would be attenuated to the level of near inoperation. \$\endgroup\$
    – Michael Karas
    Commented Jun 21, 2016 at 4:32
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A resistor is just one possible termination. You can have a look at different termination schemes in TI's Application Report (especially the AC termination).

I would guess that you have standard unit load devices. When you have 32 of them the bus works at maximum load (or on the edge of it), you then add some more load (resistors) that make it stop working.

If you can't replace at least some of the devices with 1/4 unit load transceivers, then I would try AC termination first.

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