
1.OVERVIEW OF PMC B-63983EN/02
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1.4.3 Processing Priority (1st Level, 2nd Level, and 3rd Level)
A sequence program consists of two operation parts: a high-speed
sequence part called the 1st level, which is executed every several msec,
and a normal sequence part called the 2nd level. When the model used
allows use of the 3rd level, the 3rd level sequence part is added. (See
Fig. 1.4.3 (a).)
Specifies end of 3rd level sequence
Sequence program
1st level sequence part
2nd level sequence part
3rd level sequence part
(only with model that can
use 3rd level)
Specifies end of 1st level
sequence
Division 1
Division 2
Division n
Specifies end of 2nd level
sequence
Fig. 1.4.3 (a) Sequence program structure
The 1st level sequence part is a high-speed sequence part that is
executed every ladder execution cycle. The ladder execution cycle is 4
or 8 msec, which is set in a CNC parameter. If the execution of the 1st
level program requires a long time, the overall execution time including
the 2nd level (sequence processing time) is extended. So, the 1st level
sequence part should be created so that it can be processed in a short
time where possible. The 2nd level sequence part is executed every
(ladder execution cycle × n) msec (where n is the number by which the
2nd level is divided). The 3rd level sequence part is executed when the
PMC is idle.
(1) Division of the 2nd level program
The 2nd level program must be divided to execute the 1st level
program. The order of sequence program execution is illustrated
in Fig. 1.4.3 (b), where the number of divisions is assumed to be n.
After the last division (division n) of the 2nd level program is
executed, the sequence program is executed from the beginning.
Therefore, when the number of divisions is n, the execution cycle
of the overall sequence program is expressed as the ladder
execution cycle × n msec.