Synchronous And Asynchronous Ladder Operation; Figure 11-6 Synchronous And Asynchronous Ladder Operation - Hitachi S10VE User Manual

11.3.2 Synchronous and asynchronous ladder operation

You can specify settings for synchronous or asynchronous ladder processing with remote I/O transfer input
and output. Figure 11-6 describes synchronous and asynchronous ladder operation.
When ladder processing is asynchronous, ladder processing and remote I/O transfer operations are performed
individually. For this reason, you can have remote I/O transfers operate in a shorter time. However, because
the PI/O memory changes during ladder processing in one sequence cycle, actual ladder processing and
operation might differ from what is expected.
During synchronous ladder operation, the next remote I/O transfer operation is not performed until the ladder
processing during one sequence cycle is complete. For this reason, the PI/O memory does not change during
one sequence cycle, and ladder processing is performed as expected. However, remote I/O transfer periods
become longer as ladder programs grow larger.
■ Operation in asynchronous mode
Ladder
processing
Remote I/O
operation
■ Operation in synchronous mode
Ladder
processing
Remote I/O
operation
Note: In synchronous mode, the time for one sequence cycle cannot be made shorter than the time for one cycle of the remote I/O.

Figure 11-6 Synchronous and asynchronous ladder operation

One sequence cycle
PI/O memory
Even with inputs of the same I/O number, values might differ depending on the location of ladder diagram.
One sequence cycle
PI/O memory
Temporary
Transfer operation
stops
For the same input I/O number, the value is always the same.
Conflict!
One cycle
Wait for end of ladder processing and copy
One cycle
11-5
11. Operation