YOKOGAWA STARDOM FCN-500, Engineering Manual

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<8. Detailed Description>
150
TI 34P02K35-02E Jun. 6, 2018-00
8.4.3 Combining FBD, LD and ST
Logic Designer does not allow an application to be coded in ST, which is a text-
based language, on an FBD or LD logical POU, which is graphical-based. Similarly,
it does not allow an application to be coded in FBD or LD on an ST logical POU.
However, some applications may be better implemented by combining the features
of graphical languages and text-based languages.
In these situations, application parts coded in ST can be registered as user function
blocks, and combined with application parts created in FBD or LD. This enables
creation of an application that exploit the features of both graphical languages and
text-based languages.
● Example Combining FBD, LD and ST
To illustrate how FBD, LD and ST can be combined, consider the following
application:
Calculate difference between PV values of two NPAS_PVIs.
If control mode is “a” and the calculated difference exceeds a preset value, turn
on signal “A”.
If control mode is “b” and the calculated difference exceeds a preset value, turn
on signal “B”.
In this application, the part for finding the PV values of two NPAS_PVIs is regulatory
control of analog signals, for which FBD and LD are well suited, while the part for
difference calculation, comparison against a preset value and mode comparison
comprises calculations and conditionals, for which ST is well suited.
PV1
PV2
AI1
AI2
difference = | PV1 - PV2 |
If mode = “a”
If difference ≥ preset value,
turn on signal A
If mode = “b”
If difference ≥ preset value,
turn on signal B
Regulatory control
of analog signals
Calculations
and conditional decisions
FBD, LD is suitable ST is suitable
NPAS_PVI_
2
NPAS_PVI_
1
If this application is created entirely using FBD or LD, the part for finding PV values
of NPAS_PVIs would be nicely represented but the part for the calculations and
conditional decisions would have to be implemented using Return function and
Jump function, leading to complicated coding; if created entirely using ST, the
difference calculation, preset value and mode comparisons would be simple but the
regulatory control of analog signals would not be easily comprehensible.
For an optimal solution, the calculations and conditional decisions are coded in ST
and registered as a user function block, and then combined with NPAS_PVIs
created using FBD and LD. This allows the part for regulatory control of analog
signals to be coded in FBD or LD and the part for the calculations and conditional
decisions to be coded in ST.