226 Ethernet module User’s Manual
Appendix E Computer Link/PC Link Protocol Minimum Transmission Delay Times
E
Below we present the technique for calculating the response time at the S controller
main unit for computer link protocol transmission.
(a) For fixed scan
If a fixed scan period is used, the following condition must be fulfilled:
(S controller main task scan time) > (S controller main task user program
execution time)
The computer link response text creation processing will be one of the following:
• If the value [(S controller scan time) - (user program execution time)] is larger
than the (response text creation processing time), the creation processing
can be executed within a single scan. However, due to the timing of the
arrival of the request with respect to the S controller scan timing, the time
required will be as follows:
(Response time at the S controller) = (S controller scan time) × 2
• If the value [(S controller scan time) - (user program execution time)] is
smaller than the (response text creation processing time), since the process-
ing will be performed across n scans (where n > 1), the time required will be
as follows:
(Response time at the S controller) = (S controller scan time) × (n+1)
n = int{(response text creation processing time) / (remaining scan time in a
single scan)} + 1
(b) For floating scan
If a floating scan period is used, two milliseconds are allocated in each scan for
computer link response text creation processing, and the time required will be as
follows:
(Response time at the S controller) = (S controller scan time) × (n+1)
n = int [ (response text creation processing time) / 2ms] + 1
Usage Recommendations
• There are cases where the transmission delay time will be longer than the calculated value
due to details of the actual environment used. Factors that can cause the transmission delay
time to increase include network load, EN311 load (on both the local and remote stations),
and the S controller operating state.
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