9. Settings
9-11
9.3.5 Ladder synchronous/asynchronous mode setting
The CPU module provides a function that allows remote I/O to be synchronous or asynchronous with ladder
logic processing.
In asynchronous mode, ladder processing and remote I/O transfer operate independently, allowing remote I/O
transfer to complete in the shortest possible time. However, this can mean that the PI/O memory changes state
during a given sequence cycle of ladder processing, potentially resulting in ladder processing that does not
behave as intended.
In synchronous mode, the system does not transition to the next remote I/O transfer operation until ladder
processing has completed in a sequence cycle. This means that ladder processing always proceeds as expected
without the possibility of the PI/O memory state changing during a sequence cycle, but results in slower
remote I/O transfer cycles as the size of the ladder program increases.
■
Operation in asynchronous mode
■
Operation in synchronous mode
■
Setting synchronous or asynchronous mode
For details on how to enable synchronous or asynchronous mode, see the
S10VE Software Manual Operation
Ladder Diagram System for Windows®
(manual number SEE-3-131).
Note: In synchronous mode, a sequence cycle cannot be shorter than a remote I/O cycle.
Ladder
processing
Remote I/O
operation
One sequence cycle
PI/O memory
Contention occurs!
Input from the same I/O number can yield different values at different points in the ladder diagram.
One cycle
Ladder
processing
Remote I/O
operation
One sequence cycle
PI/O memory
Copied at conclusion of ladder processing
Input from the same I/O number always yields the same value
One cycle
Temporary buffer
Transfer is stopped
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