DHRIO Frozen Input Issue

3 months into the PLC game, so please excuse my ignorance. I'm troubleshooting a level input that sporadically freezes on a PLC 5/20E. It is reading this input from a MSG instruction on a 1756 L72 Contrologix. The level on that controller is always updating normally. The message instruction comm details are DH path, Ch. A destination, node 74. I have a DH module fault on the contrologix, "no connection instance 16#ff00." "BDH2 only Node"
The manual on this module based on the conditions states to check cabling. This is where I could use some guidance. Would this be something regarding the resistor? What ohms/signals should I have coming from the logix and at the PLC? Since it's only a few wires, I'd imagine other things on this comm path would be affected as well.
I'm leaning towards a cable signal issue on this trunk line, however I'm not ruling out something in the logic going wacky. I am aware of the math overflow fault as well.

Any pointers would be greatly appreciated!!!

First. Is this a new configuration or has it been working correctly until recently?

It's been working.

Hi,

I can't answer your questions directly, but there may be a couple of things you can do to help narrow things down:

One think I always do when I'm putting clx on to an old DH+ network is to put in a hearbeat monitor. I do this for two reasons, the first is so that I know right away if my DH+ comms are down, second reason is so that if I do have a problem, I can rule out the network if my hearbeat is good. In your case this would let you separate problems with the value you are reading from the network itself.

There are several ways to create the heartbeat, but an easy way is to put a counter in your clx that is updated and periodically reset by a timer, then read that counter acc value in your plc5 and watch that acc value to be sure it is changing.

Regarding the DH+ module error, is your DH card in your I/O tree? Because unless you have RIO attached to it, it should not be in your tree. This will give you an error because it's looking for the I/O. Removing it from the I/O tree should remove that error. That doesn't mean you don't still have a cabled problem, but you'll def. get a minor module fault if you have it in your I/O tree unnecessarily.

I hope that helps.

BenL said:

3 months into the PLC game, so please excuse my ignorance. I'm troubleshooting a level input that sporadically freezes on a PLC 5/20E. It is reading this input from a MSG instruction on a 1756 L72 Contrologix. The level on that controller is always updating normally. The message instruction comm details are DH path, Ch. A destination, node 74. I have a DH module fault on the contrologix, "no connection instance 16#ff00." "BDH2 only Node"
The manual on this module based on the conditions states to check cabling. This is where I could use some guidance. Would this be something regarding the resistor? What ohms/signals should I have coming from the logix and at the PLC? Since it's only a few wires, I'd imagine other things on this comm path would be affected as well.
I'm leaning towards a cable signal issue on this trunk line, however I'm not ruling out something in the logic going wacky. I am aware of the math overflow fault as well.

Any pointers would be greatly appreciated!!!

Click to expand...

Assuming that your issue is probably with your cable (understanding that this might not be the case), then you should at least verify these things:

1. Is the cable being used the correct twinaxial cable? It should be either the Belden cable specified by AB or an exact equivalent regarding cable construction and characteristic impedance.

2. Does the cable, when disconnected, measure any faults between the conductors or between one or more conductors and ground? You'd be surprised by how many people fail to make this simple test.

3. Are the terminating resistors installed, and are they of the correct value for the application, based upon the baud rate being used, etc.?

4. Is the cable exposed to any significant sources of electromagnetic interference?

I guess it's too late to edit my post, but I wanted to add that distance limitations are also dependent upon baud rate. Your cabling requirements for DH or RIO will vary somewhat, based upon the details of the specific application.

DHRIO troubleshooting

Thank you all for the info. I will look into the cabling today. It's a strange situation, as this just recently starting happening from all accounts, which may or may not be true. The PLC 5 has two node faults on it that have been there forever apparently. So it makes it a bit harder to see what I really am troubleshooting. Here is a dropbox link to the both PLC programs. If anyone has a minute or two to peruse them for additional feedback that would be of much help!

Thanks again all!

https://www.dropbox.com/s/udwgqpyaqu7vh1b/Valves.zip?dl=0

It sounds to me like your 1756-DHRIO has both channels configured for DH+.

I can't open the Studio 5000 file to be sure; what version was it saved in ?

This would explain two of the things you are seeing: the Connection failure to the module, and the "DH Only Node" message on the unused Channel B.

Check the error counters for the module via RSLinx Classic (over the backplane from Ethernet or ControlNet, if possible) to get an idea of the number of retries and errors occurring on the network.

This is probably a busy, old, network that needs some tuning and repair.

Changed Ch. B to RIO and error stopped displaying. Thanks for the help!