Modicon Quantum: Remote I/O Rack Problem (IOs communication or Drop-out)

System Details:
We use hot redundant Modicon Quantum PLC with dual redundant communication having Remote IO racks (using CRP + CRA cards and coaxial cable). One rack is for Hot Redundant CPUs and other 12 racks are for Remote IO Drops (total 13 Drops). The model numbers of installed cards are given below
  1. CPU: 140 CPU 434 12A (486 Controller)
  2. RIO Head: 140 CRP 932 00
  3. RIO Drop: 140 CRA 932 00
  4. Software: ProWORX 32
  5. Dual redundant communication using Coaxial Cable and splitters

Problem Faced and Action Performed:
Our plant was running and tripped without any process abnormality. Most of inputs configured in different DI cards of the rack 2 actuated at the same time, which triggered the tripping. The same was diagnosed from the alarms history on HMI. No evidence of system / rack failure was found on CPU/CRA/CRP or HMI.

We replaced the CRA (RIO Drop) card of the rack 2, as this could be the single point of failure.

B. The plant was started again, but some hours later, the same nature of problem and alarm from inputs of DI cards were observed.

This time the Drop's back-plane and DI cards of actuated inputs were replaced.

C. The plant was started again and to our surprise, tripping happened again after a few hours with same alarms .

Upon further troubleshooting, PLC Status tab of ProWORX 32 an Overrun error in word #175 and word #178 were found.

Based on the errors, PV and SV versions of CRA cards of all racks were compared. Their SV versions were in the range of 1.XX. whereas the only 1 CRA card had the SV of 2.00, which was replaced initially during Action mention in para A.

Therefore, the card with SV2.00 in drop 2 was replaced with SV version 1.26.

D. After this activity the plant was started again and is running in normal state till the time of writing .

  1. What could be the possible causes of these nuisance trippings?
  2. Can the difference in PV and SV versions of CRA/CRP card, result in such trippings ? If yes, Is there any compatibility matrix.
  3. What is probable cause of Overrun Error ? Is there any relation of this error with these trippings?
  4. Do you have any case study and example of such nature of trippings?