Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 288)

Publication 1785-UM012D-EN-P - July 2005
C-2 Maximizing System Performance
Effects of False Logic versus True Logic on Logic Scan Time
The rung below—which changes states from one program scan to the next—
will change your scan time by about 400
µ s.
Other instructions may have a greater or lesser effect.
Effects of Different Input States on Logic Scan Time
You can write your logic so that it executes different rungs at different times,
based on input conditions. The amount of logic executed in logic scans causes
differences in program scan times. For example, the simple differences in rung
execution in the following example cause the program scan to vary.
NATURAL LOG
Source
Dest
N7:0
F8:20
I:000
00
LN
5
1.609438
If I:000/00 is Then the Rung is
On True, and the controller calculates the natural log.
A natural log instruction takes 409
µ
s to execute.
Off False, and the controller scans the rung but does not execute it.
It takes only 1.4
µ
s to only scan the rung.
If I:000/02 is Rungs 2 and 3 are
On Skipped
Off Executed
00
JMP
rung 1
B3:0
20 I:000
02
MVM
MVM
JMP
O:013 20
LBL
02
rung 2
rung 3
rung 4