Technical data

Note
If DB x + 1 is not kept free during the initialization, it will be
used as a controller data field without any warning if its length is
identical to that of a controller DB (49 data words); data words 20
through 48 are cleared. Otherwise the CPU goes to the stop mode.
Instead of DB data blocks, you can also use DX data blocks.
Initialization is the same as with DB data blocks.
6.32.4
OB 251: Processing the
PID Algorithm
Application OB 251 is called during cyclic program execution and processes the
PID algorithm.
Call The controller should be called after the sampling time has elapsed.
Keep to the following order:
Step Action
1 Call data block DB x + 1
2 Load input data X
k
, XZ
k
, Z
k
and YH
k
or a subset of
these
3 Convert input data to the correct format and transfer it
to DB x + 1
4 Call OB 251 (process PID controller)
5 Load the output data YA
k
from DB x + 1
6 Convert the data and transfer to the process I/Os
6
OB 250/251: Closed-Loop Control/ PID Algorithm
CPU 928B Programming Guide
C79000-B8576-C898-01
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