We have a third party PLC (modified Modicon) communicating processed data from an fiber optical gauge to DeltaV DCS. When there is any failure on the Modbus port or cable I am told that the data in DCS remains "last value". Is this true?

 

This is really a DeltaV question (or even the driver in use within DeltaV), as there may be various ways to configure the DCS to overcome this.

However, in many systems the Modbus or raw array-of-word data is static & 'refresh' is a magic background task. You need to check a device, driver or block error to understand that the data is no longer being refreshed.

The problem is assuming the value is 16-bit unsigned, what does the system set it to to indicate error? 0? 65535? There is no clear-cut universal way to signal the DCS logic that the value is no longer being updated.

OPC and other 'object systems' can return various errors when the DCS tries to fetch the data value, but a raw Modbus word is just a number.