Modbus Plus Network Problems

[Gamble 0]

Ok here goes, I've got a Modbus Plus network with about 8 nodes on it. The last node drops out all the time. It drops out at a high seemingly random frequency. I've gone over documentation several times, and I have done all the tests that I can think of several times. Here is a list of what I know. The total network cable length is <1000ft. The resistance betweens pins 2 and 3 of the drop cables is about 72 ohms. There is an open between 1 & 2 and 1 & 3 There is short between 1 and ground. The first node is terminated correctly. The last node is terminated correctly. No node in the middle of the network is terminated. The correct Belden cable is being used. Modicon Tee-Taps are being used as well as drop cables. All the Tee-Taps are in the correct direction. The Modbus indicator lights on all of the CRA modules are flashing correctly. The errorA light will come on intermittently between the modules (some modules may have it while others don't). I've looked at the signals with a scope, and it does not show qualites that I would associate with noise being induced onto the line. Does anybody have any ideas what could be going on? Does anybody know what the waveform is supposed to look like, or know any specifics of the signals (peak voltages, etc...)? Any help would be greatly appreciated.

[Modiconbob 2]

Gamble, It appears that your network itself is OK. I worked for Modicon for over 17 years an still do MB+ network certifications. If all of your testing results are as you say, then the network structure is not the issue. I do have a couple of questions: Are you taking the resisstance reading at each end where the terminator is located? And were the nodes connected or disconnected? Have you looked at the cable system with a TDR? What are the drop cable lengths from the tees? Are they ALL grounded? When you say "the last node", what are you referring to? What do you mean by "drops out"? What types of nodes are on the system? Do you have any bridge-plus devices? Have you monitored the system statistics with MBP Stat? If so, what are the results? If indeed the system structure is OK, there are three other possibilities that we usually run across: A defective piece of hardware ie: CRA that you mentioned. Electrical noise, ie: the grounding question. A "bottleneck" of data due to faulty programmng techniques ie: excessive data paths etc. Let me know if I can be of more help. I'm traveling this week so I may not be able to respond as swiftl as I normally could. If you'd like, you can contact me via my regular email and I'll give you my cell#. bobculley@deltaautomation.com Good luck and let me know what you find! Bob

[Gamble 0]

We took the resistance reading at the node farthest from the "start" (the NOM module) of the network. All the nodes were disconnected. We have not used a TDR, nor do we have access to one. The drop cables from the TEE are all probably around 3 feet or less. They are all grounded. BTW there is also about 10ft of cable between each TEE. When I say last node I'm thinking of the network as starting at the NOM module in the PLC and ending at the farthest node. (I realize that in the truest sense this is probably not the best terminology.) By drops out: If I monitor normally on bits (NC switches and buttons) on the farthest node, they will flash on and off a few times a second. All the nodes are DIO and some AIO. We have no bridge devices. We have not monitored the system with MBP Stat. Some of the issues with doing this are as follows. We have two networks; one connects the plant PLCs together and then each PLC has a network for remote racks (mostly I/O). The only PC that can get on the network is the "programing terminal" which is connected on the PLC network. To get on the network in question we'd have to drag the PC out to the plant floor, create another drop for the PC, and hook it up. (While this is certainly an option, I don't know if the powers greater than myself will want that.) Also I've used MBP Stat to some extent, but I can't say that I really know how to use it. We have tried replacing the CRA module in the problem node. It did not change anything. I tend to thing that the programing technique is OK, because in the past there were two more nodes on this network (thus more data) and it, reportedly, worked fine.