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formed in cyclic fashion, with each scan forming one cycle. The cycle time is
the time that is required for the CPU to complete one of these cycles. This
cycle includes basically five types of operation.
1, 2, 3... 1. Overseeing
2. Link Unit servicing
3. Peripheral device servicing
4. Program execution
5. I/O refreshing
The cycle time is the total time required for the PC to perform all of the above
operations.
The last of the above five operations is composed of up to four separate
components. The breakdown of this operation and the function and time
required for each operation are shown in the following table.
Operation Time required Function
1. Overseeing 2.6 ms Watchdog timer set. I/O Bus, Program
Memory, and cycle time checked.
2. Host Link Unit
servicing
8 ms max. per unit Commands from computers connected
through Rack-mounting Host Link Units pro-
cessed.
3. Peripheral device
servicing
0 ms when no devices are mounted.
0.8 ms when T is less than or equal to 13 ms.
T x 0.06 ms when T is greater than 13 ms.
(T is the total cycle time for operations 1, 2, 4,
and 5.)
Commands from Programming Devices and
Interface Units processed.
4. Program
execution
Total execution time for all instructions varies
with program size, the instructions used, and
execution conditions. Refer to 6-3 Instruction
Execution Times for details.
Program executed.
5. I/O refreshing
70 µs per input word. 40 µs per output word.
(12 pt. units are treated as 16 pt. units.)
1.3 ms per Remote I/O Master Unit + 0.2 ms
per Remote I/O Slave Unit number used.
PC Link and Special I/O Unit refresh time, as
shown in the following tables.
Input bits set according to status of input sig-
nals. Output signals sent according to status
of output bits in memory.
Inputs and Outputs in Remote I/O Systems
refreshed.
Special I/O Units serviced.
PC Link Unit I/O Refresh
Switch 7 setting I/O pts to refresh Time required
Pin 1 Pin 2
(ms)
0 0 512 8.9
0 1 256 5.7
1 0 128 3.6
1 1 64 2.8
Cycle Time Section 6-1
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