Ladder Synchronous/Asynchronous Mode Setting - Hitachi S10VE User Manual

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
Ladder
processing
Remote I/O
operation
■ Operation in synchronous mode
Ladder
processing
Remote I/O
operation
Note: In synchronous mode, a sequence cycle cannot be shorter than a remote I/O cycle.
■ 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).
One sequence cycle
PI/O memory
Input from the same I/O number can yield different values at different points in the ladder diagram.
One sequence cycle
PI/O memory
Temporary buffer
Transfer is stopped
Input from the same I/O number always yields the same value
Contention occurs!
One cycle
Copied at conclusion of ladder processing
One cycle
9-11
9. Settings