Measured values

Command 3 has a special function. It can be used to read the instantaneous current value via HART, plus up to 4 process values including their units. The number of returned process values depends on the HART slave used.

A graphical display of the values is available via the "Measured Values Display" tab in TwinCAT. 

Measured values 1:

The values are automatically updated every three seconds. This value is independent of the set polling time.

Cyclic process data

Furthermore, HART Cmd 3 can be mapped cyclically (see CoE object index 0x6080 for channel 1 or index 0x6090 for channel 2). A HART-specific set of additional process data, which can be read by the control system, can be created by ticking the “Cyclic Process Data” checkbox.

Measured values 2:
Additional PDO (Cmd 3) of the HART communication

Once the configuration has been activated, it is updated cyclically with the polling time used as time base. The acyclic service from this dialog takes values from the cyclic buffer every three seconds.