OIS41 Communication Error - 'Plant Loop #2 IN' has Errors
Voted best answer
The "C" is a fairly generic summary status for a communications problem and comes from N90STA tag for the OIS. Is the message 'Plant Loop #2 IN has Errors' from the status bytes of the OIS, if so this is quite specific and worth following up on.
Your fault finding so far indicates that the problem probably does not lie with the upstream node or the loop cabling between the upstream node and the OIS with the error. I.E. The C (and possibly the "loop 2 in" error) disappears and does not move to the down stream node when the OIS is set offline and the MCL01 is in bypass.
With regards to the timing of the "C" error, the "loop in" error alarms have some filtering so may take a minute or two to appear but not much longer.
Not sure what version of OIS software you have but the "later" OIS software has Infinet diagnostics. From the General Functions Menu select B Utilities > A PCU Configuration > C INFI-NET Diagnostics. From there you can access error counters and other diagnostics that might help.
If you have coax for the Infinet I have found that the skinny NKTL01 cables used to transition from the thick NKCL01 coax cable to the MCL01 have often been a source of problems in the OIS. The tight access for the cables seems to play havoc with the coax connectors. The problems can be quite intermittent, but as mentioned previously the loop cabling appears to be OK if the error does not move to the down stream node when the OIS is set offline.
If the problem has been there "forever" or only since the last work on the OIS maybe the problem lies with the setup of the cards. Specifically, The MCL01 has jumpers that need to be set to the media type (coax/twinax) and the MLM01 has jumpers for loop speed of the MLM receiver circuits.
There is also a ribbon cable between the 2 cards, maybe this has mechanical damage.
Other than that try replacing the MCL01 and then the MLM01 and seeing what happens. Only reason for the MCL01 first is that I don't think I have ever seen a failed NIS/MLM. As usual, be careful with the MCL as disconnecting both loops will stop communications. If you have coax just use a couple of 50 ohm coax couplers to keep the loop running, for twinax you can use a double row terminal strip or just borrow an NTCL01 from somewhere and use as a patch panel.