• Local Override is a non-target mode that instructs the blocks to revert to Local
Override when the tracking or fail-safe control options are activated.
• Out of Service mode disables the block for maintenance.
Abrupt changes in the quality of the input signal can result in unexpected loop behavior.
To prevent the output from changing abruptly and upsetting the process, select the SP-PV
Track in Man I/O option. this option automatically sets the loop to Manual if a Bad input
status is detected. While in Manual mode, you can manage control manually until a Good
input status is reestablished.
7.9.10 Application examples
Basic PID block for steam heater control
Situation
A PID block is used with an AI block and an AO block to control the flow steam used to heat
a process fluid in a heat exchanger. Figure 7-17 illustrates the process instrumentation
diagram.
PID function block steam heater controlFigure 7-17:
Solution
The PID loop uses TT101 as an input and provides a signal to the analog output TCV101.
The BKCAL_OUT of the AO block and the BKCAL_IN of the PID block communicate the
status and quality of information being passed between the blocks. The status indication
shows that communications is functioning, and the I/O is working properly. Figure
illustrates the correct function block configuration.
Foundation Fieldbus
Reference Manual 165
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