Modbus Plus framing & CRC errors


Thread Starter


The MB+ network spans 3 buildings with excellent communications in 2 of the buildings. It isn't an easy option to just pull all the cable, so is there a methodology, test software, etc that I can use to find the problem in the cable?

Any assistance here will be helpful

I see that the initial part of my message wasn't posted, so the posting lacked some context.

I am adding two nodes to an existing MB+ network and found that I am only getting a good communication to the nodes ever couple of minutes. I used MBPstat and found that those nodes were getting framing and CRC errors.

I have done the basics of looking at the cables for obvious shorts,etc., but because other devices on other locations on the same MB+ network cable work fine, I am not sure how to approach this.


Kevin Frechette

Framing errors can come from long cable lengths, ground loops or poor terminations. The CRC errors are indicating that the data is scrambled.

The first thing to check is that the network is properly terminated at both ends with a 150 ohm resistor. You can use a ohmeter to test for 60-80 ohms at a station near the end.

RS-485 networks are designed to be daisy chained or with very short drop cables from the main trunk. If your configuration is starred and branched then the maximum cable length can be shortened considerably.

I've seen an over extended network having your problems. The stations all close together on one end run fine and the one station at the extreme end runs with errors as well as the peers that he communicates with.

If the cable length is exceeded, consider putting in a pair of repeaters or running fiber between buildings.
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