Modbus Plus Comm Issues

boudreaub

Member
Join Date
Sep 2004
Posts
1
We have a rather large MB+ network consisting of two Citect HMI workstations, a PI interface, two 984-145 PLC's, two Quantum PLC's, 17 M1 processors, and two repeaters. The Citect and PI stations are conencted to the network via an SA85 card. Each network segment has a terminator on each end. In addition, each node is connected via the MBKT connector and daisy chained from one node to the next. Finally, the majority of hte network is wired using Belden 9841 cable, with only a couple of drops using Modicon's 490AA271XX cable. The longest segment we have is about 1200 feet.

The problem I'm having is mainly with the first segment of the network, between the Citect stations and the very last node on this segment before the first repeater. The last node is an M1 processor. At the Citect workstations, I will see #COM errors in lieu of the analog data coming in from this node. This indicates communications errors wiht this particular node.

I verified this in three additional ways. I first use MBPSTAT to look at the entire network, and saw that this node was showing up as unknown. In addition with MBPSTAT, I tried going to node statistics to see what was going on. What I saw was about one failure in every ten successes. Looking at what MBPSTAT was telling me, I saw several CRC and framing errors, indicating a possible cabling or connector issue. the cabling and the connector for this drop has been replaced. In fact, the cabling for the entire network has been replaced and re-rerouted two years ago to make it easier to expand on the existing network.

I additionally attempted to connect to the node using Modsoft from the Citect stations. I kept getting an error. FInally, I used a laptop with a PCMCIA adapter to try to talk to the node by lifting one of the connectors from one of the Citect stations and connecting it to the laptop. Once again, I got the same errors.

If I take the same laptop and systematically disconnect connectors from existing nodes and re-connecting them to the laptop. As I got closer and closer to the affected node, the communications errors would go away.

When I would remove the connector from the affected node and replace it on my laptop, all communications errors would go away. I'd go back to the Citect workstations and be able to see the laptop perfectly with no errors.

This is a situation that has been ongoing now for two years. I'd re-terminate the connector at the affected node, and the communications would sometimes return. I've also replaced the connector (many times), the terminators on both ends, the communications adaptor (many times), the M1 processor a couple of time, and even the I/O backplane a couple of times.

Currently, I'm guessing I may have a possible ground loop here. I say this because the communications issues come and go so intermittently. The length of the error is also very intermittent (from a few minutes to several days). In addition, the affected node is grounded via a ground wire back to its main power panel. A couple of other nodes on this segment are also grounded in a similar manner. Some other nodes on this segment are grounded via local building steel.

My questions are: Could I be right in regards to the ground loop idea? If not, could anyone else give me some pointers as to where to go from here? Also, has anyone else ever had similar experiences with MB+?

Thank you in advance to anyone who may answer this post.

Brian Boudreau
 

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