Yaskawa YASNAC PC NC PLC Programming Manual - Manual (Page 19)

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YASNAC PCNC PLC Programming Manual Chapter 4: Sequence Control Method
4.1 Differences In Operation
There are two types of operation modes in the sequence control-relay sequence and PLC
sequence.
4.1.1 Relay Sequence
All devices are processed simultaneously.
4.1.2 PLC Sequence
Sequence control by PLC is executed sequentially by the software, which differs from the
ordinary control by relay circuits in which processing is executed simultaneously. Due to this
characteristic, the sequence control by PLC results in considerably different operation from
ordinary relay circuit processing. When developing programs, this must be completely under-
stood.
Devices are processed sequentially and the ladder is executed repeatedly in a fixed period.
This period is called the scan time.
Fig. 4.1.2.1 PLC Sequence Program
The PLC sequence ladder above operates in the following sequence. The operation is not
processed simultaneously.
€ The status of contact A is read.
ó The read status is output to internal relay B.
ì The status of contact A is read.
ö AND operation is executed between the status of contact A and the status of NC contact of
relay B.
ú The result of AND operation is output to internal relay D.
As the result of sequential processing, internal relay D is never turned ON.
However, if the same ladder is executed in relay sequence, relay D is momentarily turned ON
(one-shot operation).
As discussed above, programming must always be carried out taking into consideration that pro-
cessing by the PLC is executed sequentially.