98
1
2
3
4
Overseeing
Peripheral Device
servicing
Instruction
execution
Input refreshing
Output refreshing
1.26 ms (Fixed)
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.
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.
0.29 ms + 0.07 ms times N where N =
number of input and output words – 1.
Watchdog timer set and program
memory and I/O bus checked.
Commands from Program Devices and
Interface Units processed.
Instructions executed.
Reading data input from input terminals
and writing the results of instruction
execution to output terminals.
The scan time can be obtained by adding the four scan time components
identified above. An adequately short scan time is important to ensure effi-
cient, error-free operation.
Within the PC, the watchdog timer measures the scan time and compares it
to a set value. If the scan time exceeds the set value of the watchdog timer,
an error is generated and the CPU stops.
Even if the scan time does not exceed the set value of the watchdog timer, a
long scan time can adversely affect the accuracy of system operations as
shown in the following table.
Scan 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 ALARM indicator on the CPU lights and SR 1809 turns ON.
100 and 130
130 or greater ERROR indicator on the CPU lights and the system halts.
6–3 Calculating Scan Time
The PC configuration, the program, and program execution conditions must
be taken into consideration when calculating the scan 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
subsection shows some basic scan time calculation examples.
6–3–1 Single PC Unit
Configuration: A single C20K CPU.
Program: 300 addresses.
Instructions Used: LD and OUT.
The equation for the scan time from above is as follows:
Scan time = Overseeing time
+ Input/output refreshing
+ Peripheral device servicing time
+ Instruction execution time
Watchdog Timer and Long
Scan Times
Calculations
Calculating Scan Time Section 6–3
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