You can preserve an OPC item’s last read by enabling the Latch Data option. Should a communication failure occur, Data links to unlatched items display a series of question marks (?????) while Data links to latched items display the last data polled. This feature is very useful when you need to create reports about your process and require data at all times. You can also use this option to preserve the last values on the screen after a communication failure occurs.
Example
Your OPC server communicates with remote devices by radio transmission. You know that radio transmission is not always reliable (particularly during inclement weather); however, you must provide daily reports from your process hardware.
By enabling the Latch Data option for your items, you ensure data is always available for reports.
NOTES:
- Latch data is also available from the OPC server. Since data quality is returned from the OPC server, the server can latch the item’s quality. Enabling latched data in the driver forces the quality to reflect a latched state and changes the actual data quality returned by the OPC server.
- Analog Register and Digital Register blocks do not support latched data. Consequently, we do not recommend using these blocks with the Latch Data option.
