Rockwell Automation 1755-OF8 GuardPLC Controller Systems - Manual (Page 177)

Publication 1753-UM001C-EN-P - March 2010 177
Configure Peer-to-Peer Communication Chapter 17
The time actually needed for communication adds to the CPU cycle
time. A short Communication Time Slice limits the communication
time to a low value. This prevents the CPU cycle time from being
noticeably influenced by network occurrences. Although a
Communication Time Slice well above the minimum value may result
in cycle time on the local machine slowing down a bit if network
traffic is heavy, it is not necessarily negative.
If you are transferring safety I/O over the network, you need a
Communication Time Slice high enough to guarantee that the
communication are completed every cycle. If it takes more than one
cycle to read/write safety I/O, your safety time will need to increase to
compensate.
If you are only transferring status data over the network, then a lower
Communication Time Slice is permissible, because it leaves more time
in the cycle for your program to run. It’s likely to be acceptable even
if it takes more than one cycle to read the status.
Check the CPU short-term diagnostics for any Time Slice expired
entries and increase the Communication Time Slice if necessary,
before the application goes into regular operation. In the Statistics of
the Control Panel, Number of Time Slices higher than 1 also indicate a
Communication Time Slice that is too short. Number of Time Slices
indicates the number of cycles it took for communication to complete.
The maximum Communication Time Slice depends on the application
and is calculated as follows.
WDZ
≥
Communication Time Slice (max) + Application
Execution Time
In other words, the Communication Time Slice plus Application
Execution Time must not exceed the Watchdog Time.
EXAMPLE
If the controller on page 176 has 10 connections, the
minimum Communication Time Slice is:
CTS
min
= 10 x 1 ms + 4 ms = 14 ms.
CTS
min
is increased by 6 milliseconds to provide a
safety margin.
CTS
min
= 20 ms
With a Watchdog Time of 500 ms, this leaves 480 ms
for the application to be executed.