Termination resistor for modbus rtu

I have one pm583-eth communicating over modbus rtu(COM1) with one slave having baud rate of 9600.Is it mandatory to use termination resistor at slave end as well at the plc .Should pull up & down resistors be activated in COM1 port of the plc.

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arunvt1992 Rank: 653

Asked 8 months ago

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Hi,

It is not mandatory to activate the bias resistors on the COM-1 port. I have never activated them in any of the installations I have done and have never experienced communication issues which required activating the bias resistors.

Most important thing to make sure when dealing with RS485 communication is to use Twisted pair cable only. The Twisted pair cable helps eliminate noise in the line (Common mode noise).

It is a good practice to,

1) Connect the ground wire as well

2) Connect the cable screen (shield) to instrument earth at one end only

Termination resistors are only required if signal reflections cause communication issues .The reflections may increase when the bus length is long or if higher baud rates are used.For short bus lengths , termination resistors are not mandatory and may not provide any marked improvement in communication.

Regards
Rahil

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by arunvt1992 Rank: 653 on 10/10/2018 1:41:18 AM | Like (0) | Report

hi im getting message time out error in com_mod_mast.Also through a rs485 converter and modscan im getting the modbus message time out error.All the communication parameters are the same for master and slave.

by rahilmohammed Rank: 61 on 10/10/2018 2:03:37 AM | Like (0) | Report

Hi,
Since you are getting timeout message on modscan as well, you need to verify the following and resolve it using Modscan first and then proceed to establish communication with PLC as Master.

1) Baud rate, Slave ID, no of stop bits and parity specified for both sides are
identical
2) Check whether the correct register address is specified
3) Verify whether data type specified is correct
4)If reading floating point address, specify the correct word order and length
5)Verify if Function code used is supported by slave

6) In COM_MOD_MAST block starting address shall be Slave Red Addr-1 , if first register in slave has register no 400001 (HR)

by rahilmohammed Rank: 61 on 10/10/2018 2:06:12 AM | Like (0) | Report

Also in COM port modbus setting, Telegram option must be selected for RS485 communication.

by arunvt1992 Rank: 653 on 10/10/2018 6:43:48 AM | Like (0) | Report

I checked all the parameters both are the same..while i tried swapping the cables, the "ERNO" output of mod_mast remains 0 which means "No error" while the DONE_MOD output of the mod_mast is not completed.

by arunvt1992 Rank: 653 on 10/10/2018 6:48:12 AM | Like (0) | Report

pls ignore the above comment..even in modscan im not getting the values ,it indicates time out error

by rahilmohammed Rank: 61 on 10/10/2018 7:23:15 AM | Like (0) | Report

Hi,

It is very difficult to resolve field side issues via the forum. An experienced engineer should be able to identify why modscan is unable to communicate with the slave (Is your RS485 converter reliable???).
Please take support from your seniors to first communicate via modscan.Later if needed, the forum members will be able to help better when you are working on establishing communication via the PLC.

By the way what device is the slave?

by arunvt1992 Rank: 653 on 10/10/2018 9:08:17 AM | Like (0) | Report

It is a btu calculator from Landis &gyr .UC50 which has a modbus communication module Wzu-485-mod

by rahilmohammed Rank: 61 on 10/10/2018 10:27:38 AM | Like (0) | Report

Hi,

I am assuming that you have prior experience on establishing communication with modbus slaves using Modscan software.Hence, I feel you should concentrate on troubleshooting from slave side.
In one site where I worked in, every 5th SOCOMEC PMU had a faulty RS485 module.

by arunvt1992 Rank: 653 on 10/11/2018 12:15:10 AM | Like (0) | Report

Hi , the converter is working fine I was able to read values from the plc as slave , but when I try to simulate values from mod SIM, in com_ mod mast the error is 0 which means no error while done mod is not energized..why is it happening like this

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rahilmohammed Rank: 61

Answered 8 months ago

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when there is impedance mismatch between Hardware & Transmission line, then really termination/bias resistor of 120ohm is required. How to find Impedance mismatch. As low level, Data quality is not good, lot of data loss will be there.

Please refer below link
https://blog.opto22.com/optoblog/rs-4...

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Vinoth Sankar Rank: 112

Answered 8 months ago

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Termination resistor for modbus rtu

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