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Data Link I/O Response Time Section 8-3
Case 4
The following diagram illustrates the data flow that will produce the maximum
data link I/O response time when the cycle time of the PLC at node #1 < the
communications cycle time and the cycle time of the PLC at node #7 < the
communications cycle time.
There are three points shown in the diagram above where processing is
delayed, increasing the data link I/O response time.
1,2,3... 1. The input arrives in the PLC just after I/O refreshing, causing a delay of up
to one cycle before the input is read into the PLC.
2. At point A data from the previous data exchange is still being transferred,
so the new data cannot be exchanged, causing a delay of up to one com-
munications cycle time. Furthermore, the data exchange then occurs just
after the PLC at node #1 passes the token that makes it the polling node,
causing another delay of up to one communications cycle time before the
data is transferred in data link processing.
3. The data transferred in data link processing arrives at the PLC at node #7
while data exchange is taking place, so the data will not be read into the
PLC until the next data exchange, causing a delay of up to 1 to 2 cycle.
The maximum number of words that can be exchanged in a single data ex-
change is approx. 7,800 words for CS/CJ-series PLCs, approx. 3,700
words for CVM1 and CV-series PLCs, approx. 4,000 for C200HX/HG/HE
PLCs, and approx. 1,200 words for CQM1H-series PLCs.
Input device
Program
Program
Input
Input ON delay
1 cycle
Out
p
ut
Output device
Output ON
delay
1 cycle
Communications
cycle time
Data link transmission
Controller Link Unit
transmission processing
PLC at node #7
(1)
I/O refresh
Data exchange
Controller Link Unit
transmission processing
PLC at node #1
Data link I/O response time
X
X
(2)
(3)
A
X
Previous communications
cycle data