Publication 1785-UM012D-EN-P - July 2005
C-4 Maximizing System Performance
Because the first program scan takes 28 ms, the STI actually occurs 12 ms into
the second program scan (28 + 12 = 40, which is the time for the second STI
to occur). This example points out that when the STI time period is different
than the program scan time, the STI occurs in different places in the program
scan. Also note that, due to fluctuations in program-scan times, multiple STIs
may be executed during one scan and no STIs during other scans.
Effects of Housekeeping Time
In PLC-5 controllers, basic housekeeping takes 3.5 ms. If it takes the
controller 21.8 ms to execute a ladder program, the overall program scan time
is 25.3 ms. Any increase in housekeeping affects your program scan.
The following activities can increase housekeeping time:
• editing while in remote run mode
• putting block-transfer modules in the controller-resident chassis
• using the global status flag files
Editing While in Remote Run Mode
The online editing times for ladder programs are as follows:
For this Editing Operation And this Type of
Program
The Times are
Accept Rung (after inserting, modifying,
or deleting a rung edit)
other than the edited file 0.35 ms per 1000 words
no labels 3 ms + 0.35 ms per 1000 words
with labels 3.5 ms + 0.35 ms per 1000 words
Test Edits of the program (impacts one
program scan)
0.2 ms to change the status of
edits from TEST to UNTEST or
UNTEST to TEST
Assemble Edits no edits pending 0.35 ms per 1000 words
edits pending, no labels 2.0 ms + 1.5 ms per 1000 words
edits pending, with labels 2.0 ms + 1.9 ms per 1000 words
IMPORTANT
Editing programs online also delays the execution of PIIs
and STIs.
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