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Chapter 9 PLC Operation
9-4
(2) Causes of congestion errors at normal scan
Congestion errors may occur at normal scan because of the following three possible reasons. In particular when using a
periodical scan program and an interrupt scan program together, care must be taken to create the program in such a way
that the total scan time does not exceed the congestion check time.
(a) When only a normal scan program is used
The scan time exceeded the congestion check time because the time required for one scan was too long.
Figure 9.5 Congestion error at normal scan (a)
(b) When both a normal scan program and a periodical scan program are used
The congestion check time was exceeded because the periodical scan program was executed and the normal scan
time became longer.
Figure 9.6 Congestion error at normal scan (b)
(c) When both a normal scan program and an interrupt scan program are used
The congestion check time was exceeded because the interrupt scan program was executed due to an interrupt
input and the normal scan time became longer.
Figure 9.7 Congestion error at normal scan (c)
(3) Continuation of operation after a congestion error occurred
When the special internal output bit R7C0, which specifies whether the operation should continue after a congestion
error occurred, is turned on, the normal scan executes the scan until the end regardless of the congestion check time, and
after executing the END scan processing, executes the normal scan from the beginning again.
Figure 9.8 Operation when operation continuation at congestion error is set
However, note that this setting does not stop the execution of the scan when a congestion error occurred even when an
infinite loop is formed within the normal scan by the JMP instruction.
10 ms
The periodical system interrupt
is executed by interrupts at every
10 ms regardless of whether or
not there is a periodical scan or
interrupt scan.
The congestion check time can
be set to 20 ms to 2550 ms using
peripheral devices by the users.
Normal
scan
END scan processing
Program execution
Periodical system processing
Congestion check time
Normal scan time (normal scan only)
RTI
INT0
10 ms
RTI
INT0
N
ormal
scan
END scan processing
Program execution
Periodical system
processing
Periodical program
Congestion check time
Sc
an tim
e
Periodical
scan
10 ms
N
ormal
scan
END scan processing
Program execution
Periodical s
y
stem
p
rocessin
g
Congestion check time
Sc
an tim
e
Interrupt analysis
processing
Pro
g
ram execution
I
nterrupt
scan
10 ms
N
ormal
scan
END scan processing
Program execution
Periodical system processing
Congestion check time
Scan time