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

Solution:
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.
 
Keywords:
ABCLX, Request Size too big, OPC, OID  

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