Applies To:
  • CitectSCADA 5.xx, 5.50, 5.50 Service Pack A, 5.50 Service Pack B, 5.50 Service Pack C, 6.00

When opening pages in my project that display tags talking to an I/O device which uses an OID based driver, such as ABCLX or OPC, I get the error: "Driver ABCLX/OPC on port "PortName" reports "request size too big error". I have not defined any arrays in my plc or citect project that is greater than the block size for the driver (i.e. 254 bytes)

The error may be caused by the fact that the project ID of the Citect project is too large. OID based drivers uses the project ID as part of a unique identifier when reading from and writing to tags. The OID (Object ID) is a 32 bit unique identifier for a tag that is calculated as:

The project ID is a signed integer, therefore the first bit is used for this purpose. This tells us that the largest possible value that can be used as the Project ID before we get a wrap-around is 2^(10-1)-1 or 511. It is strongly recommended that a Project ID is within the range of 1 – 511. Using a project ID within this range should get rid of this error.

Also see KB Q1180 and Q3755 for other possible solutions.

ABCLX, Request Size too big, OPC, OID