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Termination resistor on RJ50 cable for Modbus protocol over a RS485 supervisory network

We are looking at communicating with a device over Modbus using a RS485 cable, with a RJ50 connector on a cRIO-9035. Has anyone else ever done this before, and if so did you need a termination resistor on the cRIO end?

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Hey GBN,

 

Yes, in general that is possible. Be aware that the cRIO-9035 supports Modbus both on the RS485 port (RJ50) and an Ethernet port (commonly called "RJ45"). Be sure to select the "serial" one (=RS485) in case you have both options, e.g. when setting up the connection in LabVIEW.

 

Yew, you will need termination for your RS485 bus. I found a brief overview here: RS485 Termination and Serial Troubleshooting Wizard. Basically, you need a resistor connecting your lines, on both far ends of the cable. Only in the case that you have one-way communication form a transmitter on one end of the cable, you can omit the terminator(s) at this transmitter.


Ingo – LabVIEW 2013, 2014, 2015, 2016, 2017, 2018, NXG 2.0, 2.1, 3.0
CLADMSD
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Hi ikaiser, thanks for the response. I've made some progress with this. As it turns out you are right that in most cases a termination resistor is necessary for both ends, however for low speed applications (I am using a 9600 baud rate) it might not be required.

 

It depends on how long the propagation delay caused by the impedance mismatch between the transmission line and the hardware is. If it is significantly less than one pulse and therefore doesn't affect the signal quality, it's not needed. This is confirmed in my application as I am able to communicate without the termination resistor.

 

Nevertheless, I would be interested to know if anyone else has actually implemented this. Our problem is the physical characteristics of the RJ50 connector needed for the RS485 port on the cRIO doesn't exactly help in adding a termination resistor. I suppose it's always possible to cut open the cable, add a termination resistor, then reconnect the wiring to original connector. I was just wondering if anyone else had found a more elegant method of implementing this really. Something like an appropriate adaptor RJ50 interface that includes a termination resistor would be ideal, however it probably only exists in my dreams...

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We have 20+ systems that use the onboard RS485 port on cRIO-9035s.

A termination resistor is required at the instrument end only. We only have short runs (10-20ft) to the instruments even when daisy chained. Use only daisy chain connections and don't fan the system out from the first connection point for the bus.

We have terminated the RJ50 cable at a binding post on the cRIO enclosure where the +send/recieve are joined together and the -send/receive are joined forming a 2 wire RS485. 2 wires go from there to the instrument where the termination resistor is installed.

We have had reliability problems with the onboard serial port as it seems to be sensitive to power glitches. An RS 485 plug in module is more reliable.

 

 

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