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Chapter 9 PLC Operation
9-8
9.1.4 Relationship of Each Scan Type
When three types of scan occur at the same time, scan is executed in the order of periodical scan, then interrupt scan, and
then normal scan.
Figure 9.15 Relational diagram of scan operation
Table 9.3 List of interrupt label
Interrupt label Cause of startup Interrupt label Cause of startup
INT0 Interrupt every 10 ms INT20 Counter 1 on-preset match
INT1 Interrupt every 20 ms INT21 Counter 1 off-preset match
INT2 Interrupt every 40 ms INT22 Counter 2 on-preset match
INT16 Interrupt of interrupt input 1 INT23 Counter 2 off-preset match
INT17 Interrupt of interrupt input 2 INT24 Counter 3 on-preset match
INT18 Interrupt of interrupt input 3 INT25 Counter 3 off-preset match
INT19 Interrupt of interrupt input 4 INT26 Counter 4 on-preset match
INT27 Counter 4 off-preset match
Low High
Execution priority
Interrupt scan Periodical scanNormal scan
Normal
scan
Ladder program
END scan
Processing
Normal scan
Interrupt scan
10 ms10 ms10 ms10 ms
Periodical scan
Periodical system processing
INT16
INT17
INT18
:
:
INT27
10 ms periodical
program
20 ms periodical
program
40 ms periodical
program
Interrupt analysis
processing
Interrupt program
Interrupt
scan
Periodical
scan