Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 114)

Performance and Theory of Operations
Chapter 7
7-9
Effects of False versus True Logic on Scan Ti me
The rung below—which changes states from one program scan to the
next—will change your scan time by about 400
ms.
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 processor calculates the natural log. A natural-log instruction takes
409
µ
s to execute
Off false and the processor scans the rung but does not execute it. It takes only 1.4
µ
s just to scan the rung.
Other instructions may have a greater or lesser effect.
Effects of Different Instructions on Logic Scan Time
Some instructions have a much greater effect on logic scan time than
others based on the time it takes to execute each instruction.
Program scan time is also affected by the basic construction of your ladder
rungs. The sizes of rungs and the number of branches in each can cause
the scan time to fluctuate greatly.
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 different amounts of logic executed
in the logic scans causes differences in program scan times. For example,
the simple differences in rung execution in the following example cause
the logic scan times to vary.
00
JMP
Rung 1
B3:0
20 I:000
02
MVM
MVM
JMP
O:013 20
LBL
02
Rung 2
Rund 3
Rung 4