Permitted and Actual Mode

The input parameters of are function block are used by the algorithm in conjunction with the state of the function block application to determine if the algorithm can achieve the target mode of operation established for it. The target mode attribute as origianlly defined by ISA50 is the part of the mode parameter that indicates what mode of operation is desired for the block. Under certain operating condition, a block may not be able to function in the requested mode. In such cases, the actual mode attribute reflects the mode of operation it is able to achieve. Comparison of the actual against the target indicates whether the target was achieved.

As detailed in Control Loop Foundation, the allowed target mode values for an instance of a block are defined by the permitted mode attribute of the mode parameter. Thus, the modes available for controlling a block may vary with each block instance and are assigned during block configuration. Interface devices and temporary devices may use the permitted mode to limit operator changes in target mode. Function blocks limit changes in target mode to those allowed by the permitted mode attribute. To help an operator select the normal mode of operation from those available, a normal mode attribute is provide in the mode parameter. This attribute may be used by a human interface application to help guide the operator in setting the operation mode of a block.

The operator modes for the PID block and the impact of the mode selection on the source of setpoint and output are summarized in table below

The Actual mode attribute may take on a value other than that specified by the Target mode attribute. This occurs when an internal condition or input status indicates that the mode of operation requested by the operator through the Target mode attribute cannot be achieved. Under normal operating conditions, the Target and Actual mode attributes match. The Actual mode attribute changing to Local Override (LO) indicates that the output tracking or an internal application, such as auto-tuning, is setting the block output. Similarly, if the path to the process is lost, then the Actual mode attribute will change to Initialization Manual (IMan) as summarized

When the Actual mode of a PID is shown as IMan, this indicates that the path to the process is not complete. The path to the process can be broken if a downstream block is taken out of Cascade mode. For example, in a single- loop application, the path of the PID output to the process will be broken by changing the mode of the analog output block from Cascade to Automatic. The path to the process may also be broken by a physical condition such as the failure of an actuator that is manipulated by the PID. When the Actual mode of the PID is IMan, the PID cannot adjust the manipulated process input. Under this condition, the PID output follows the value provided by the downstream block through the back calculation input.