
B-63983EN/02 1.OVERVIEW OF PMC
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As the amount of the 1st level sequence part increases, the amount
of the 2nd level sequence portion executed within the ladder
execution cycle decreases. As a result, the number of divisions n
increases, which increases the overall execution time including
the 2nd level (sequence processing time). Therefore, the 1st level
sequence program part should be minimized where possible. The
division number of 2nd level may be indefinite because of
changing of the working condition of functional instructions in 1
st level and 2 nd level.
1st level
2nd level
3rd level
Ladder execution cycle (4 or 8 ms) Ladder execution cycle (4 or 8 ms)
Division 1
Division 2
Ladder execution cycle(4 or 8 ms)
Last division n
3rd level
processing
Fig. 1.4.3 (b) Sequence program execution order
(2) 1st level sequence part
High-speed sequence operation. Only high-speed sequence
processing such as processing of a pulse signal with a short signal
width in time is performed.
These signals include emergency stop and feed hold signals.
(3) 3rd level sequence part
The 3rd level sequence processing is performed during the
remaining time from the end of the last division (n) of the 2nd
level until the 1st level processing restarts (see Fig. 1.4.3 (b)).
It is possible to program the 3rd level, but the execution cycle
period of time for processing the 3rd level sequence part is not
guaranteed to maintain program compatibility with conventional
models. Therefore, the 1st and 2nd level sequence parts should be
programmed without using the 3rd level sequence part.