Allen-Bradley 1747-SN - Minimizing the Scan Time

M0–M1 Files and G Files B–7
Publication 17476.6 - July 1996
Minimizing the Scan Time
You can keep the processor scan time to a minimum by economizing
on the use of instructions addressing the M0 or M1 files. For
example, XIC instruction M0:2.1/1 is used in rungs 1 and 2 of the
figure below, adding approximately 2 ms to the scan time if you are
using a SLC 5/02 Series B processor.
XIC instructions in rungs 1 and 2 are addressed to the M0 data file. Each of
these instructions adds approximately 1 ms to the scan time (SLC 5/02
Series B processor).
1
] [
B3
12
] [
M0:2.1
1
( )
B3
14
] [
M0:2.1
1
( )
B3
10
2
In the equivalent rungs of the figure below, XIC instruction M0:2.1/1
is used only in rung 1, reducing the SLC 5/02 scan time by
approximately 1 ms.
These rungs provide equivalent operation to those of figure A by substituting
XIC instruction B3/10 for XIC instruction M0:2.1/1 in rung 2. Scan time is
reduced by approximately 1 ms (Series B processor).
] [
B3
12
] [
B3
10
( )
B3
14
] [
M0:2.1
1
( )
B3
10
1
2
The following figure illustrates another economizing technique. The
COP instruction addresses an M1 file, adding approximately 4.29 ms
to the scan time if you are using a SLC 5/02 Series B processor.
Scan time economy is realized by making this rung true only
periodically, as determined by clock bit S:4/8. (Clock bits are
discussed in appendix B of the SLC 500t and MicroLogixt 1000
Instruction Set Reference Manual, Publication 1747-6.15.) A rung
such as this might be used when you want to monitor the contents of
the M1 file, but monitoring need not be on a continuous basis.
[OSR]
B11
0
COP
COPY FILE
Source #M1:4.3
Dest #N10:0
Length 6
] [
S:4
8
S:4/8 causes the
#M1:4.3 file to update
the #N10:0 file every
2.56 seconds.