Applies To:
  • CitectSCADA
  • OPC driver version

If an OPC device is defined with more than 32766 variable tags assigned to it then the OPC driver will not subscribe to all the tags correctly.

When CitectSCADA starts the I/O server will report that the unit has come online successfully.
If any read requests are sent to the device it will report Unit Offline. The I/O server will then bring the unit online again and the process repeats.

It is unwise to create a single OPC device with this number of variable tags, if nothing else the device is likely to take a very long time to come online due to the time it takes to subscribe to each tag.

In this situation it is recommended to create multiple I/O devices that all address the same OPC server. Each of these devices should have less than 32767 tags defined against them.

This article supersedes Q3400.

OPC unit offline hardware alarm