Global and indirect points are very useful and are usually a mandatory part of most application programmes. It is seen that in the event of a LAN failure or any failure of the indirect point being updated, the indirect point will be flagged as “Old Data”.
When this occurs any process using the indirect point as a variable, say, calculation extension, event extension will freeze at the last status/value before the old data on the indirect point was detected. Even though other parameters are still valid, the calculation and the event extensions will remain frozen, and some occasion remain in this state even when the old data is cleared from the source. Any subsequent process that uses the calculation or event definition will also be frozen. Any PID DDC Modules, Floating DDC Modules and 2 Position DDC Modules that uses the indirect point that has been flagged as ‘old data’ will force those DDC modules to “Fail Safe” mode.
ALL I/NET Software and controller LAN (CLAN) based controllers
LAN failure or any failure of the indirect point being updated.
There are various tools available within I/NET to filter 'old data' from subsequent control processes. These include:
- Filtering by Calculation AVG operator function (including point types AI, DI and DO)
- Filtering by Direct Digital Control (DDC) Modules using either the Relay or HiLo modules
Click Here to download a document which details these methods.
For further details and recommendations when using Global and Indirect points, refer to the following Lessons Learned Article #5487.