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Within the PC, the watchdog timer measures the cycle time and compares it to a
set value. If the cycle time exceeds the set value of the watchdog timer, a FALS
9F error is generated and the CPU Unit stops. WDT(94) can be used to extend
the set value for the watchdog timer.
Even if the cycle time does not exceed the set value of the watchdog timer, a long
cycle time can adversely affect the accuracy of system operations as shown in
the following table.
Cycle time (ms) Possible adverse affects
10 or greater TIMH(15) inaccurate when TC 016 through TC 511 are used.
(Accuracy when using TC 000 through TC 0015 not affected.)
20 or greater 0.02-second clock pulse (SR 25401) not accurately readable.
100 or greater 0.1-second clock pulse (SR 25500) not accurately readable and
Cycle Time Error Flag (SR 25309) turns ON.
200 or greater 0.2-second clock pulse (SR 25501) not accurately readable.
6,500 or greater FALS code 9F generated regardless of watchdog timer setting
and the system stops.
Online Editing When online editing is executed from a Programming Device, operation will be
interrupted for a maximum of 80 ms and interrupts will be masked to rewrite the
user program. No warnings will be given for long cycle times during this interval.
Check the effects on I/O response time before editing the program online.
When bits 00 to 07 of AR 25 contain the password code of “5A,” online editing will
be disabled and the CPU Unit will be in standby status while the Online Edit Dis-
able Bit (AR 2509) is ON. The Online Edit Standby Flag (AR 2510) will be ON
while the CPU Unit is in standby status. Processing will be executed when AR
2509 is turned OFF. (AR 2510 will be turned OFF, too.)
Caution Editing the program online can cause delays in I/O responses with no warnings
being given from the system for the long cycle time produced by editing online.
Before editing online, make sure that delays in I/O responses will not create a
dangerous situation in the controlled system.
6-2 Calculating Cycle Time
The PC configuration, the program, and program execution conditions must be
taken into consideration when calculating the cycle time. This means taking into
account such things as the number of I/O points, the programming instructions
used, and whether or not peripheral devices are employed. This section shows
some basic cycle time calculation examples. To simplify the examples, the
instructions used in the programs have been assumed to be all either LD or OUT.
The average execution time for the instructions is thus 0.156 µs. (Execution
times are given in the table in 6-3 Instruction Execution Times.)
Watchdog Timer and Long
Cycle Times
Calculating Cycle Time Section 6-2