Omron SYSMAC CS Series Operation Manual page 94

Difference between I/O Refreshing in Customizable
CPU Unit
Customizable
Counter Unit
74
Note Even if the same interval is set using the constant cycle time function, syn-
chronization actually will not last long even when it does occur because the
intervals are not that precise. This means that a period of overlap will be fol-
lowed by a period of non-overlap.
■ Other Cases
The I/O refresh timing on both sides may or may not overlap consecutively
under certain conditions in items (1) to (3) below.
1. When the cycle times of both the CPU Unit and the Customizable Counter
Unit are constantly changing.
2. When the cycle times of both the CPU Unit and the Customizable Counter
Unit are stable yet different.
3. When the constant cycle time function is used to set a different cycle time
interval for the CPU Unit and the Customizable Counter Unit.
When the I/O Refresh Timing on Both Sides Overlaps Consecutively:
When the CPU Unit cycle time - the Customizable Counter Unit cycle time <
the I/O refresh time of the Customizable Counter Unit in the CPU Unit
When the I/O Refresh Timing on Both Sides Will Not Overlap
Consecutively:
When the CPU Unit cycle time - the Customizable Counter Unit cycle time >
the I/O refresh time for the Customizable Counter Unit in the CPU Unit
Even if the I/O refresh timing on both sides overlaps momentarily, it will not
overlap in the next cycle in these cases. This means that the I/O can be
refreshed (data exchanged) within the maximum output response time (2
cycles) of the PLC. (See Figure 3.)
Note The I/O refresh time for the Customizable Counter Unit under the CPU Unit is
equivalent to the time it takes the CPU Unit to access the shared memory.
I/O refresh time for the Customizable Counter Unit in the
CPU Unit
If this difference is longer than the I/O refresh time
for the Customizable Counter Unit in the CPU Unit,
Start
there are no consecutive overlaps.
1 cycle
I/O refresh time for the
Customizable Counter Unit
CPU Unit Customizable
−
cycle time Counter Unit
cycle time
Thus, overlapping does not occur consecutively.
Figure 3
Section 4-5
I/O refresh overlap (the
Customizable Counter Unit
does not refresh the I/O.)
I/O refresh time for the
>
Customizable Counter
Unit in the CPU Unit