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Termination resistor for Serial Card Series 2 RS485 half-duplex 1300' / 400 M total, 3 nodes queried, 19200 baud

Does anyone deploying serial RS485 networks include a 120 Ohm terminating resistor at the DeltaV Serial card terminals?

I've been communicating via Modbus RTU over RS485 for years and have never used any (either end) but the runs were shorter (more like 200 M) and baud rates were 9600.

Thanks,

John

John Rezabek

• 27 May 2026 6:50 PM
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3 Replies

• Chris Kominar

  

  • 27 May 2026 7:54 PM

  We have multiple serial RS485 networks communicating Modbus RTU without a terminating resistor at the DeltaV Serial card. One of the networks is ~1000M with a 38400 baud rate. Another is ~500M with a 19200 baud rate. All of these are connecting to a ProSoft card on the PLC side.

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• Matt Forbis

  

  • 28 May 2026 2:58 PM

  Termination is one of those small overlooked things that generally will not bite you until it does. In general, the slower the speed, the shorter the cable, the better the quality of cable, shielding, terminations, and the less number of devices on the segment, the better communications are in general. As I mainly support AMS Device Manager and there are lots of MUX installations with RS-485, we see it popup occasionally where termination is the solution to give reliable communications.
  
  If it were me for 1300' and 3 nodes at 19200, I would probably lean towards terminating, but it all depends on whether you like gambling or not. Some of the most difficult problems to debug can be traced back to power, grounding and proper terminations.

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• jgrunschel

  

  • 3 Jun 2026 7:11 PM

  I can confirm that 1,050 feet at 9600 baud with unshielded cable does not work, with or without a terminating resistor. I didn't install the cable. Unfortunately, I didn't have the funds to re-pull shielded cable, but got funding to add a controller and pull network cables. Funding is weird, but that is off-topic.

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