As an operator changes the mode of a function block, it is important that this change not cause a sudden change in plant operation. In the ISA50 mode implementation, the status in the backward path plays an important role in providing bumpless transfer. For example, the actual mode of an analog output block will not transfer to a requested Cascade target mode until a handshake is completed between the PID and the analog output block. This handshake consists of the following steps:
- When the analog output block target mode is changed to Cascade, the status in the back calculation connection changes from Not Invited to Initialization Request.
- On detection of Initialization Request in the back calculation input status, the PID sets its output to match the analog output block setpoint value provided through the back calculation connection. Once initialization is complete, the output status is set to Initialization Acknowledge.
- On detection of Initialization Acknowledge in the PID connection, the analog output block changes the actual mode attribute to Cas. It then starts to use the PID output provided through the connection input as its setpoint.
As detailed in Control Loop Foundation, the use of a handshake between blocks ensures that the PID output matches the analog output block setpoint when the actual mode of the analog output block transitions from Auto to Cascade mode. Thus, within the PID function block mode must be evaluated before the main algorithm is executed as illustrated in the figure below where the order of execution is indicated by sequence number.
If the blocks are configured to execute at a fast rate then the handshake between blocks may occur so fast that it may not be apparent. Nonetheless, it is a very important feature of the function blocks used in control applications.
The information provided in the back calculation connection continues to be used during normal operation. For example, the high and low limits status and limited setpoint value passed through the back calculation connection are used to alert the PID that a change request was not fully acted on by the downstream block. The PID is designed to compensate for limiting in a downstream block. If the error is such that the reset is going to drive the PID output further into the limit condition, then the last value of the reset contribution may be maintained or calculated, based on the limited value, to avoid reset windup. Normal integral action is restored when changes in the PID integral contribution to the output will move the output away from a limit condition.