Hitachi NJI-350B manual Periodical Scan, Congestion error at periodical scan 10 ms

Chapter 9 PLC Operation

9.1.2 Periodical Scan

(1)Definition and operation

This scan executes interrupt programs (periodical scan programs) while the CPU is operating with a fixed cycle time (10 ms, 20 ms, or 40 ms) specified by the users.

Enter the periodical scan program to be executed between instructions INT0 and RT1 if it should be started up with a 10 ms cycle time, and between INT1 and RT1 if it should be started up with a 20 ms cycle time.

The periodical system processing is executed every 10 ms regardless of whether or not there is a periodical scan program.

Normal scan Periodical scan

Periodical system processing (including interrupt analysis processing)

Periodical scan

Program execution

Periodical system processing

Scan every 10 ms Periodical program

Scan every 20 ms Periodical program

Scan every 40 ms Periodical program

Normal scan

10 ms

20 ms

Interrupt	Interrupt	Interrupt
INT1	INT1	INT1
RTI	RTI

Figure 9.9 Operation of periodical scan (in case of INT1)

(2)Causes of congestion errors at periodical scan

If there are periodical scans at every 10 ms as well as scans at every 20 ms or 40 ms, a congestion error occurs and the scan is stopped if the periodical scan at 10 ms is started up again before all the periodical scans are completed (i.e., the periodical system processing at INT0 to INT2 does not end within 10 ms).

Periodical system processing	10 ms	10 ms
Scan every 10 ms

10 ms

		Periodical program			Stop			Stop
Periodical scan		Scan every 20 ms
		Periodical program
		Scan every 40 ms
		Periodical program
Normal	END scan processing
							RTI
scan	Normal scan program
				INT0		INT0
							INT1
		(Example 1)			(Example 2)

INT0

(Example 3)

Stop

RTI RTI

INT1 INT2

Before the periodical scan program at	Before the periodical scan program at
every 10 ms ends, the periodical	every 20 ms ends, the periodical
interrupt at 10 ms is started up again.	interrupt at 10 ms is started up again.

Before the periodical scan program at every 40 ms ends, the periodical interrupt at 10 ms is started up again.

Figure 9.10 Congestion error at periodical scan (10 ms)

Similarly, when executing with a periodical scan at every 20 ms or with a combination of periodical scans at every 20 ms and 40 ms, a congestion error occurs if the periodical scan at 20 ms is started up again before all the periodical scans are completed (i.e., the periodical system processing at INT1 to INT2 does not end within 20 ms). Finally, when using a periodical scan at every 40 ms, a congestion error occurs if the periodical scan at 40 ms is started up again before all the periodical scans are completed (i.e., the periodical system processing at INT2 does not end within 40 ms).

9-5