Rockwell Automation 1747-SN Remote I/O Scanner - Manual (Page 119)

Publication 1747-UM013B-EN-P - January 2005
M0 - M1 Files and G Files B-7
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.
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.
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 500 Instruction Set Reference Manual,
Publication 1747-RM001.) 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.
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
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
[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.