204
Data Link I/O Response Time
Section 8-3
Note If the total number of data link words is greater than the maximum number of
words that can be exchanged per data exchange, the maximum data link I/O
response time will be cycle time of PC at node #7
×
4.
In case of the CQM1H, however, the cycle time will be incremented by the
value obtained from dividing the total number of data link words by the maxi-
mum number of words (1,200 words) per time.
For example, if the total number of data link words is 5,000, the result will be
as shown below.
5,000/1,200 = 2.5 (approx. 3)
Cycle time of PC at node #7
×
(3+3)
Noise may increase I/O delays.
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 PC at node #1 < the
communications cycle time and the cycle time of the PC 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 PC just after I/O refreshing, causing a delay of up
to one cycle before the input is read into the PC.
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 PC 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.
Input device
Program
Program
Input
Input ON delay
1 cycle
Output
Output device
Output ON
delay
1 cycle
Communications
cycle time
Data link transmission
Controller Link Unit
transmission processing
PC at node #7
(1)
I/O refresh
Data exchange
Controller Link Unit
transmission processing
PC at node #1
Data link I/O response time
X
X
(2)
(3)
A
X
Previous communications
cycle data
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