YOKOGAWA STARDOM FCN-500, Engineering Manual

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<9. Advanced Engineering >
207
TI 34P02K35-02E May 31, 2017-00
9.2.4 Blocked NPAS_POU Status Propagation
If incorrect application programming results in incorrect upstream to downstream
propagation of status or incorrect downstream to upstream read back of status, it
may affect control.
● Blocked Status Propagation or Read Back
If a control application is incorrectly programmed so that upstream to downstream
propagation of status information or downstream to upstream read back of status
information is blocked, upstream and downstream exchange of status information
fails and correct control operation will be affected.
For purposes of explanation, consider the control loop example described in Section
9.2.3, "NPAS_POU Status Propagation" and suppose that there is a missing
connection between RB_OUT of NPAS_PID_2 and RB_IN and NPAS_PID_1. As the
state of NPAS_PID_2 is not passed to NPAS_PID_1, even if the cascade
connection is open, NPAS_PID_1 continues with PID calculation without switching
to IMAN mode. Moreover, as SINT is not passed to NPAS_PID_2, NPAS_PID_2
cannot use the MV value of NPAS_PID_1 as its own SV value.
● Examples of Application Errors
Some examples of application errors are briefly described below.
1. No Read back connection
If there is no read back connection, downstream status information of course cannot
be passed upstream. Read back connection must be done always.
2. Calculation application error
In calculations involving Value of CData_REAL type data, status information is
sometimes lost.
In the example shown in the figure below, the logic extracts Value from the output (of
CData_REAL data type) of NPAS_PID_1 using NPAS_CDR_DESTR, performs
some calculation and then returns the data as CData_REAL data type.