Hitachi NJI-350B manual Interrupt Scan, Continuation of operation after a congestion error, Stop

Chapter 9 PLC Operation

(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.

	Periodical system processing			10 ms		R7C0 On/R7C1 On: Continue
		Scan every 10 ms
		Periodical program				R7C0 Off/R7C1 On: Stop
Periodical		Scan every 20 ms
		Periodical program
scan
		Scan every 40 ms
		Periodical program
Normal		END scan processing	INT0		INT0
scan		Normal scan program			Periodical interrupt (10 ms
		Periodical interrupt
					periodical interrupt restarts)
				Congestion check time

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.

Normal scan	Interrupt scan	Periodical system
		processing
	Interrupt analysis processing
			INT16
			INT17
	Interrupt analysis
	processing		INT20
			Normal scan
	Execution of
	interrupt program

	10 ms
	RTI
	RTI
INT20	INT16
	INT17

RTI

RTI

INT16

INT20

INT17

RTI RTI

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.

Periodical system processing

Interrupt	Interrupt analysis
	processing
scan
	Interrupt scan
Normal	END scan processing
scan	Program execution

10 ms		The interrupt contact of INT16 is turned
		on during the execution of the INT16
		program.
			Stop
		Interrupt contact on (INT16)
	Interrupt contact on (INT16)

Figure 9.13 Operation of interrupt scan

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