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The first three operations immediately after power application are performed
once each time the PC is turned on. The then on the operations shown
above are performed in cyclic fashion, with each cycle forming one cycle.
The cycle time is the time that is required for the CPU to complete one of
these cycles. This cycle includes four types of operation.
1. Overseeing
2. Input/Output refreshing
3. Peripheral Device servicing
4. Instruction execution
Cycle time = Overseeing time + Input/output refreshing + Peripheral
Device servicing time + Instruction execution time
1 Overseeing Watchdog timer set and program memory
and I/O bus checked.
1.6 ms (Fixed)
2 Peripheral Device
servicing
Commands from Program Devices and
Interface Units processed.
T = ( 1 + 3 + 4 ) x 0.05.
T <= 1, execution time = 1 ms.
T > 1, round off in units of 0.5 ms
(e.g.,1.65 ms rounds to 1.5 ms)
No peripherals connected = 0 ms.
3 Instruction
execution
Instructions executed. Total of execution time for each instruction.
Varies with program size, the instructions
used, and execution conditions. Refer to
6-4 Instruction Execution Times
for details.
4 Input refreshing
Output refreshing
Reading data input from input terminals
and writing the results of instruction
execution to output terminals.
0.51 ms + 0.03 ms times N where N =
number of input and output words – 2.
The cycle time can be obtained by adding the four cycle time components
identified above. An adequately short cycle time is important to ensure effi-
cient, error-free operation.
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,
an error is generated and the CPU stops.
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) becomes inaccurate.
100 or greater 0.1-second clock pulse generator SR 1900 may
malfunction.
Between
100 and 130
ALARM indicator on the CPU lights and SR 1809 turns ON.
130 or greater ERROR indicator on the CPU lights and the system halts.
1, 2, 3...
Watchdog Timer and Long
Cycle Times
Cycle Time Section 6-2