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