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How do I troubleshoot a copper Modbus Plus network physical installation?

Published date: 17 July 2019

Issue:
There is no communication on my Modbus Plus network. How can I troubleshoot the lines to make sure I do not have a connection problem or another issue with the cabling?

Product Line:
Quantum PLC
Premium PLC
Momentum PLC

Environment:
EcoStruxure Control Expert (Formerly Unity Pro)
Concept
ProWorx

Resolution:
The following are some basic troubleshooting tips to check a copper MB+ physical network installation. Good cabling practices should always also be employed in routing and installing MB+ cabling:

1 Ensure only Modicon cabling and cabling components are used.
2 Ensure no two adjacent nodes have less than 10 ft of physical cable between them (except if using Momentum product with cluster mode components).
3 Ensure there are no splices.
4 Ensure the topology is purely linear (branching is not allowed).
5 Ensure taps are properly sided left-to-right, left-to right. (The grounding lug on the standard taps can be utilized to determine proper siding).
6 Ensure all nodes are properly grounded.
7 Ensure no more than 32 nodes exist on the network without a RR85 repeater (usage of a RR-85 repeater allows up to 64 nodes max on the network).
8 Ensure the total network length does not exceed 1500 ft without usage of a RR-85 repeater (3 RR-85 repeaters can be utilized to connect up to 4 segments to make a total maximum network length of 6000'.
9 Check the resistance of the MB+ network. Disconnect all nodes from the network then measure resistance between the white and blue leads at any point. The resistance should be approx. 60 Ohms (derived from two 120 Ohm resistors in parallel). While spec is 60-80 Ohms, If 68 or more Ohms is read, then check the network for loose connections or damaged ferules on taps. Read resistance directly across a tap to check for resistance. Virtual continuity should be read. If more than negligible resistance is read, reseat connections. If this does not reduce resistance, replace the suspect connector or tap. If 120 Ohms of resistance is read, check cable for a break or loss of end termination at either side of the network.
 

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