Chapter 9 PLC Operation

(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.

Periodical system processing

10 ms

Normal

END scan processing

 

scan

Program execution

 

Normal scan time (normal scan only)

Congestion check time

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.

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.

Periodical Periodical system

scan processing Periodical program

Normal

END scan processing

scan

Program execution

10 ms

RTI RTI

INT0INT0

Scan time

Congestion check time

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.

Periodical system processing

10 ms

Interrupt

Interrupt analysis

 

processing

 

scan

 

Program execution

 

 

 

Normal

END scan processing

 

scan

Program execution

Scan time

 

 

 

 

Congestion check time

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.

Periodical system processing

10 ms

Normal

END scan processing

 

scan

Program execution

Scan time

 

 

Congestion check time

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.

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Hitachi NJI-350B manual Causes of congestion errors at normal scan, Scan time Congestion check time, RTI RTI INT0INT0