If a fieldbus valve loses air, or a fieldbus valve positioner loses power, the valve will either go fully open or fully closed. This state is determined by the valve itself and is often termed its shelf position or air fail position. This state is different from the Fault State parameter (FSTATE_VAL) of the Analog Output function block when it is assigned to a fieldbus valve.
By default the Analog Output function block's FSTATE_VAL and FSTATE_TIME parameters are set to 0. This causes the block to hold the last value when communication on the fieldbus is lost. It is highly recommended that you configure the FSTATE_VAL parameter to correctly set the valve position if fieldbus communications to the valve cease for the time period that you configure with the FSTATE_TIME parameter. Communication to a device is lost when a fieldbus function block is downloaded. Follow these steps to configure these parameters in Control Studio:
Assign the AO block to a fieldbus device.
This step is very important as the FSTATE_VAL and FSTATE_TIME parameters only appear in the parameter list after the block has been assigned to a fieldbus device.
Double-click IO_OPTS and enable Fault State to value.
Configure the FSTATE_VAL and FSTATE_TIME parameters.