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Cycle Time/High-speed Processing Section 6-1
6-1-7 Background Execution
Background execution can be used to reduce fluctuations in the cycle time.
Background execution is supported only by CS1-H, CJ1-H, or CJ1M CPU
Units, or CS1D CPU Units for Single-CPU Systems.
Table data processing (such as data searches) and text string processing
(such as text string searches), require time to execute, and can create large
fluctuations in the cycle time due to the extended amount of time required to
execute them.
Background execution (time slicing) can be used to execute the following
instructions over several cycles to help control fluctuations in the cycle time.
The PLC Setup enables setting background execution for each type of instruc-
tion.
Note The CS1G/H-CPU@@ (-V1), CJ1@-CPU@@, and CS1D (for Duplex-CPU
Systems), CPU Units do not this function.
• Table data processing instructions
• Text string processing instructions
• Data shift instructions (ASYNCHRONOUS SHIFT REGISTER only)
Setting background execution for the above instructions can help control tem-
porary increases in the cycle time.
Applications
Background execution can be used for large quantities of data processing,
such as data compilation or processing, that is required only at special times
(e.g., once a day) when reducing the effect on the cycle time is more impor-
tant than the speed of the data processing.
Procedure
1,2,3... 1. Set the PLC Setup to enable background execution for the required in-
structions.
2. Set the communications port number (logical port number) to be used for
background execution in the PLC Setup. This port number will be used for
all instructions processed in the background.
Note One port is used for all background execution. Background execution
for an instruction can thus not be started if background execution is
already being performed for another instruction. Use the Communi-
cations Port Enabled Flag to control instructions specified for back-
Communications cycle time × 210 ms × 2 ---
Cycle time of PLC at CPU Unit #2 × 120 ms × 1 Faster by 20 ms
(20 ms × 1)
Output ON delay 15 ms ---
Total (data link I/O response time) 94 ms Faster by 32.5 ms (26%
faster)
MAX MAX
CS1 CPU Unit
Long execution
time
Longer cycle time
when MAX is
executed.
CS1-H or CJ1-H
CPU Unit
Execution
only
started.
Executed over
several cycles
using time slicing.
Background
execution
↓
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