Input tag configuration

Input tags are used to receive data from PLC nodes. A tag is an input tag if function code 1, 2, 3, 4, or 65 is specified in Location3. For more information, see the Location3 section of this manual: Location3 (data type and function code).

If no "triggertag" is specified in the extended descriptor (ExDesc) attribute of the input tag, then the associated PLC will be scanned at a given frequency. The frequency is specified using the Location4 point attribute in conjunction with the /f parameter on the startup command line of the interface. The input tag is said to be scan-based in this case.

If a "triggertag" is specified in the extended descriptor (ExDesc) attribute of the input tag, then the associated PLC will be scanned only when a new value is sent to the snapshot of the triggertag. The input tag is said to be event-based in this case.

Whenever a complete response fails to be received from a PLC before a configurable timeout period has expired (see the /to=x startup command-line parameter), IO TIMEOUT will be written to the affected tags. Normally, increasing the timeout period does not help because the problem is usually related to incorrect hardware configuration. For example, if the PLC node in the Location2 point attribute is invalid, then a response may not be received by the interface.

If a communication error occurs that is not associated with a timeout, then BAD INPUT will be written to the affected tags instead of IO TIMEOUT. For example, BAD INPUT will be written to the input tags if any of the PLC exception responses that are listed in Appendix E occur for serial-based Modbus communication.

If the Scan field of an input tag is turned off while the interface is running, SCAN OFF(digital state 238) will be written to the input tag.

There are several other digital states that can also be written to input tags. These must be handled on a case-by-case basis.

Optimization

Due to the nature of the Modbus application protocol, a single request may result in a response with data for up to 2,000 tags. To optimize performance the interface will place tags into groups based on common key attributes so that the maximum number of input tags can be updated by a single response. The methodology for optimized input tag configuration can be found in Appendix C. Tag optimization.