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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.
20 or greater 0.02-second clock pulse not accurately readable.
100 or greater 0.1-second clock pulse not accurately readable and Cycle
Timer Error Flag (25309) turns ON.
200 or greater 0.2-second clock pulse not accurately readable.
6,500 or greater FALS code 9F generated regardless of watchdog timer setting
and the system stops.
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 tak-
ing 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
0.6 µs. (Operating times are given in the table in Section 6-3.)
6-2-1 PC with I/O Units Only
Here, we’ll compute the cycle time for a PC with a CPU01-E, or 03-E CPU
Unit that controls only I/O Units, eight on the CPU Rack and five on a 5-slot
Expansion I/O Rack. In this PC configuration, there is also a Programming
Console mounted to the CPU that needs to be taken into consideration. The
PC configuration for this would be as shown below. It is assumed that the
program contains 5,000 instructions requiring an average of 0.94 µs each to
execute.
CPU Rack
Expansion I/O Rack
12-point Output Units
8-point Output Unit
16-point Input Units
8-point Input Units 8-point Output Units
Calculating Cycle Time Section 6-2
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