<9. Advanced Engineering >
● Example of Correct Status Propagation
In the example given in Section 9.2.3, "NPAS_POU Status Propagation," two
NPAS_PIDs are connected in cascade.
If the application is correctly programmed, the control modes and operations of the
two NPAS_PIDs will be interlocked as shown below.
Outputs result of PID calculation to SV value of PID_2.
Performs PID calculation using MV value of PID_1 as SV value.
Stops PID calculation as cascade connection is open.
The MV value tracks the SV value of PID_2.
Continues PID calculation. The SV value can be changed freely.
Restarts PID calculation , and outputs SV value to PID_2.
Performs PID calculation using MV value of PID_1 as SV value.
When transiting from state 1 to state 2, the upstream NPAS_PID_1 switches to AUT
IMAN mode, and stops PID calculation.
Moreover, its MV value tracks the SV value of NPAS_PID_2.
When transiting from state 2 to state 3, NPAS_PID_1 restarts PID calculation, and
NPAS_PID_2 uses the MV value of PID_1 as its SV value.
This series of operation is closely related to the propagation of upstream status
information downstream and the read back of downstream status information
upstream.
When transiting to state 2, NPAS_PID_2 stores information about an open cascade
connection condition in RB_PID2, which is connected to its RB_OUT terminal.
Specifically, bit 24, which represents “CND: CoNDitional” in the status bitmap shown
below, is set to 1.
As RB_PID2 is connected to the RB_IN terminal of NPAS_PID_1, downstream CND
(CoNDitional) information is propagated upstream to NPAS_PID_1, which then
switches to AUT IMAN mode.
Moreover, as RB_PID2 also contains the SV value of NPAS_PID_2, its own MV
value tracks this value.
Next, when transiting to state 3, the CND bit of RB_PID2 is reset to 0, and bit 19
(MINT: Master INiTialize request) is set to 1.
NPAS_PID_1 accepts the MINT (Master INiTialize request) from NPAS_PID_2, and
begins preparation to restart PID calculation.
After preparation is completed, bit 18 (SINT: Settled INiTialize) of Status of the OUT
terminal is set to 1.
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