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Oct
03

Commissioning Control Systems – Increase/Decrease Selection

The analog output block action on loss of power, that is, fail open or fail closed, is defined in Foundation Fieldbus devices through the configuration of the IO_OPTS parameter. Also, as addressed in Control Loop Foundation – Batch and Continuous Processes, this parameter is also used in DCS systems that have adopted the function blocks defined by the Fieldbus Foundation. An example of the dialog provided in the DeltaV control system to change this parameter is illustrated below.

If Increase to Close is selected, 100% of output will result in an output of 4 mA, that is, the valve will fail open on loss of current or the wired connection being broken. By correctly setting Increase to Close to match valve action on loss of current, the setpoint value will always reflect the implied valve position in the field device.