As the ISA50 function block team considered how to improve plant operations through a more comprehensive implementation of mode in the control system, they realized it would be important that the control blocks, such as the PID, be designed to automatically take into account up-steam and downstream conditions that impact control operation. Thus, inputs and output parameters of function blocks used in measurement, calculation and control were defined to consist of a status attribute and a value attribute. The status attribute in the output parameter of a block used for measurement provides an indication of the quality of the measurement. For example, if the current signal exceeds the A/D converter operating range (e.g., there is an open circuit or a short circuit in the wiring) then in the block output a status attribute of Bad will be provided with the measurement value.
All function blocks in Foundation Fieldbus devices fully supported input and output status. Also, some Distributed Control Systems, such as DeltaV, support the use of status. For example, the OUT parameter of the analog input block, AI Block, is defined as a data structure consisting of a value attribute and a status attribute. These parameters are always communicated and used together. If a transmitter is scheduled for maintenance, then the operator can “red tag” the measurement by changing the block mode to Out-of-Service. In the Out-of-Service mode, the OUT status is automatically changed to Bad – Out of Service to indicate that the measurement value should not be used in calculation or for control.
As detailed on Control Loop Foundation, the status attribute as implemented in Foundation Fieldbus devices consists of eight bits. The most significant two bits are used to indicate the quality of the measurement: Bad, Uncertain, Good – Control, and Good – Measurement. The last two bits of the status attribute, the lower-priority bits, are used to indicate high or low limit, no limit, or constant. The four middle bits of the status attribute are used to show why the quality is good, bad, or uncertain.
In the case of a sensor reading, or other control output moving through the control loop from the sensor to the actuator, the status information is conveyed on a forward path. In the case of an actuator or PID function cascade setpoint, a backward path carries status information about constrained movement back to the function block that is requesting the movement. Also sent backwards is a calculated value which the requesting function block can use to balance its output. This flow of information is illustrated below.
The mode of operation that may be achieved, which ISA50 defined as the actual mode, is determined by the status of the input and output of a function block and the requested mode, the target mode. Thus, implementation of the ISA50 mode requires that the function blocks used for measurement, calculcation and control be designed to generate, propagate and use input and output status. The benefit of such a design is that the control can automatically takes into account upsteam and downstream conditions without operator intervention.