300 Tags Going IOP in Centum VP Continuously


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We have Yokogawa Centum VP DCS. 300 tags are frequently going IOP and then recover in a split second. This is occurring frequently. Vendor told us that there is noise in V-net coupler. so we changed it. Still problem persists.

BTW We have 11 FCS and RIO modules. Problem is in Modbus tags of FCS08 and FCS 09.


This happens when Modbus communication breaks. Vnet coupler has nothing to do with it.

Here's what you can do.

1. Check the physical connection between DCS and client to make sure connection is okay. (no loose connections etc.)

2. Check the length and type of the cable to see if it is within specs.

3. If you are using Modbus RTU, see if you can reduce the baud rate. For me, 19.2kbps, which is the default, works best.

4. Finally, if the problem is not resolved, try increasing the Response Timeout (default = 4 sec) and Number of Retries Upon Response Timeout (default = 1) in the properties of the Modbus communication card. Increasing these settings to, say, 10s and 5 will surely resolve the problem.
FCS08 is connected to FCS09 by V net . In FCS09 modbus is installed which communicates with PLC. PLC is communicating with FCS09 via Modbus .AI,AO,DI,DO signals are sent to DCS from PLC .The tags which are going IOP are in FCS08. If Modbus communication breaks then other AI,DI,DO signals would have been affected. But thats not the case.

Tags that are in FCS08 which is configured in Global switch are going IOP.
If 1/2 duplex and the Com Stats are about 50% good then the Host is sending out the next poll before the slave releases the com lines from the previous response.


This requires further clarification. You didn't mentioned about global switches before.

You said Modbus communication between PLC and FCS09 is healthy. And Tags going to IOP actually exist in FCS08 and are being read in FCS09 over Vnet (or its other way around?). Are you using AREAOUT block for inter-FCS communication? Please confirm if I am getting it right?

You said tags going to IOP are configured in global switches. That is also confusing. Global switches are just switches which cannot go to IOP. Only function blocks with a PV value (like PVI/PID) can go to IOP. I have never seen a global switch in IOP state.

Anther thing to check is scan transmission definition of FCS08 and FCS09. This is defined here.
SysView > FCSXXXX > Configuration > StnDef > Scan Transmission tab.

Make sure you have defined adequate buffer size for incoming and outgoing traffic, otherwise global switches will not work. See Builder Definition Items in Help for further details.