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Chapter 9 PLC Operation
9-6
(3) Continuation of operation after a congestion error
If a congestion error occurs when the special internal output bit R7C1, which specifies whether the operation should
continue after a congestion error, is turned on, the execution of the periodical scan is stopped and the periodical scan is
executed from the beginning again. If the operation continuation specification for the normal scan is Off when this
happens, the scan stops as a congestion error at a normal scan. If the operation continuation specification for the normal
scan is On, only the periodical scan continues to be executed in the event of a periodical congestion error. Care must be
taken because the normal scan is not executed under this condition.
Figure 9.11 Operation when operation continuation at congestion error is set
9.1.3 Interrupt Scan
(1) Definition and operation
If there is an input to an input terminal assigned to an interrupt input, or there is an input to an input terminal assigned to
a counter input and the current counter value exceeds the preset value while the CPU is operating, interrupt programs
(interrupt scan) corresponding to them are started up. An interrupt scan caused by an interrupt input executes interrupt
programs from INT16 to19 to RTI instructions. An interrupt scan due to a corresponding interrupt caused by the counter
current value executes the interrupt programs from INT20 to INT27 to RTI instruction.
If an interrupt caused by another factor is input during the execution of an interrupt scan, the next interrupt scan is
started up at the point when the interrupt scan being executed is completed. Also, if two or more interrupts are input
during the execution of an interrupt scan, the interrupt scans are started up in order from the smallest INT number at the
point when the interrupt scan being executed is completed.
Figure 9.12 Operation of interrupt scan
(2) Causes of congestion errors at interrupt scan
An interrupt scan congestion error occurs during the interrupt scan processing when an interrupt of the same number is
entered again.
In addition, a normal scan congestion error occurs if interrupt inputs are frequently entered because a normal scan cannot
be executed.
Figure 9.13 Operation of interrupt scan
INT0
10 ms
INT0
END scan processing
Normal scan program
Periodical
scan
Normal
scan
Scan every 10 ms
Periodical program
Scan every 20 ms
Periodical program
Scan every 40 ms
Periodical program
Periodical system processing
R7C0 On/R7C1 On: Continue
R7C0 Off/R7C1 On: Stop
Congestion check time
Periodical interrupt
Periodical interrupt (10 ms
periodical interrupt restarts)
RTI
INT20
INT17
INT16
INT17
INT16INT20
INT16
INT20
INT17
RTI
RTI
RTI
RTI
RTI
10 ms
Normal scan
Periodical system
processing
Interrupt analysis processing
Interrupt scanNormal scan
Interrupt analysis
p
rocessin
g
Execution of
interrupt program
10 ms
N
ormal
scan
END scan processing
Program execution
Periodical system processing
Interrupt analysis
processing
Interrupt scan
Stop
I
nterrupt
scan
The interrupt contact of INT16 is turned
on during the execution of the INT16
program.
Interru
p
t contact on
(
INT16
)
Interrupt contact on (INT16)