Principles of operation

For proper interface operation, the user must configure input points (input tags) and/or output points (output tags) on a PI home node. Input tags are used to receive data from PLC nodes. Data is received either at a given frequency or after a value is sent to a "trigger" tag. Output tags are used to send commands to a PLC. A command is sent to a PLC after a value is sent to a "source" tag or after a value is sent to the output tag itself, depending on the configuration of the output tag. All values that are written to the snapshot or archive use the system time from the PI home node. If a communication error occurs while attempting to read data from a PLC, the interface will attempt to re-establish communication until it is successful.

At startup, the interface scans the PI Point Database for all associated points and builds its own point list. During runtime, the interface continues to check the PI Point Database for point updates and modifies its point list accordingly. If the scan field of any point on the point list is set to zero, the point is removed from the point list. The point is added once again after the scan field is turned back on. If neither a fixed scan rate nor a valid trigger tag is found for a given point, the point will not be added to the list.

The interface is designed to optimize data transfer and minimize communication traffic by collecting input data into groups. Input points that are configured with the same function code, scan rate, and PLC destination node are grouped together. When the amount of data that is requested for a given group exceeds the maximum data transfer size (up to 250 bytes) a new group is created. The proper use of PLC memory can greatly enhance the efficiency and overall data throughput of the interface.

UniInt Failover

This interface supports UniInt failover. For more information, see the PI Universal Interface (UniInt) Framework User Guide.