Omron SYSMAC CS Series Operation Manual page 93

Difference between I/O Refreshing in Customizable
Exchanging Data between a Customizable Counter Unit and the CPU Unit
Problem
CPU Unit
Customizable
Counter Unit
A CS1W-H@@@@ Customizable Counter Unit exchanges data with the CS-
series CPU Unit in an asynchronous system using I/O refreshing timing on
both sides. (See Figure 1.)
Customicable Counter Unit
Cycle time
Common processing
Status processing
Program execution
I/O refresh
Peripheral service
Figure 1
• The Customizable Counter Unit refreshes its own I/O by reading the
shared memory inside it.
• The CPU Unit, on the other hand, refreshes its own I/O by reading the
shared memory in the Customizable Counter Unit.
This allows the Customizable Counter Unit and the CPU Unit to exchange
data (I/O refreshing). The CPU Unit, however, always has priority in accessing
the shared memory in the Customizable Counter Unit.
If the I/O refresh timing in the Customizable Counter Unit and the CPU Unit
synchronizes by chance, then the Customizable Counter Unit will not refresh
its data because the CPU Unit has priority in accessing the shared memory.
This means that the Customizable Counter Unit may not be refreshed for sev-
eral cycles.
The following problem may occur.
■
When the Same Constant Cycle Time Is Set for Both the CPU Unit and the
Customizable Counter Unit
When the constant cycle time function is used to set the same cycle time
interval for the CPU Unit and the Customizable Counter Unit, the I/O refresh
rate for both will overlap in consecutive cycles if the cycles ever become syn-
chronized. This means that the Customizable Counter Unit may not refresh
the I/O for several cycles. (See Figure 2.)
1 cycle
1 cycle
Figure 2
CPU Unit
The CPU Unit has
priority in accessing
shared memory.
Shared
memory
Asynchronous refresh
I/O refresh
Section 4-5
Cycle Time
Common processing
Status processing
Program execution
I/O refresh
Peripheral service
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