Omron SYSMAC CS Series Operation Manual page 198

I/O Communication Characteristics
Note
PROFIBUS DP Cycle <
PLC Cyclic Refresh
PLC Cycle
CIO Refresh
Output data refresh
Input data refresh
PROFIBUS Cycle Cycle n
PROFIBUS DP I/O data exchange cycle > PLC cyclic refresh results in the
following I/O transfer characteristics.
• Not all output data is guaranteed to be transmitted over the PROFIBUS
network (referring to the figure above: Output data marked with "O" is
overwritten by "O+1" and therefore lost).
• Input data is always sent to the PLC CPU.
1. PLC cyclic refresh times may vary, depending on the execution of the user
program. If variation is too large, this may result in I/O data being lost.
2. To prevent output data not being transmitted over the PROFIBUS network,
the PROFIBUS DP cycle time must made at least two times the PLC cyclic
refresh time. This can be accomplished by changing the Target Token Ro-
tation time in the Master DTM Configuration User Interface (see section 6-
4-2 Setting the Bus Parameters).
The figure below shows the effects of the cyclic refresh rules for the case in
which the PROFIBUS DP I/O data exchange cycle < PLC cyclic refresh.
Unit requests
cyclic refresh
Unit requests
cyclic refresh
Instruction Instruction
Execution Execution
O O+1
I I+1 I+2
Cycle Cycle
n+1 n+2
PROFIBUS DP I/O data exchange cycle < PLC cyclic refresh results in the
following I/O transfer characteristics.
• All output data sent by the PLC CPU is transmitted over the PROFIBUS
network.
• Not all Input data is transferred to the PLC CPU (referring to the figure
above: Input data marked with I+1, I+4, I+6 and I+8 are lost).
Note To prevent input data not being transferred to the PLC CPU, the PLC cyclic
refresh time must be at least two times the PROFIBUS DP cycle time.
Unit requests
cyclic refresh
Unit requests
cyclic refresh
Instruction
Execution
O+2
I+3 I+4
Cycle Cycle Cycle
n+3 n+4 n+5
Section 6-5
Unit requests
cyclic refresh
Instruction Instruction
Execution Execution
O+3 O+4
I+5 I+6 I+7
Cycle Cycle Cycle
n+6 n+7 n+8
I+8
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