Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 158)

Special Programming Techniques
Chapter 11
11Ć11
Unequal Block Lengths - Consult the user’s manual for the block transfer
module of interest for programming guidelines when setting the block lengths to
unequal values. Our Publication Index (publication SD499) lists the specific
manuals.
WARNING: When the block lengths of the bidirectional block
transfer instructions are set to unequal values, the rung
containing the alternate instruction must not be enabled until the
done bit of the first transfer is set. If they are enabled in the
same scan, the number of words transferred may not be the
number intended, invalid data could be operated upon in
subsequent scans or analog output devices could be controlled by
invalid data. Unexpected machine operation could occur with
possible damage to equipment and/or injury to personnel.
Section C
Special Program Techniques
In this section you will read:
How to program a one-shot
How to program a leading edge one-shot
How to use a manual restart
How to program cascading timers
The one-shot programming technique is used for certain applications to set a bit
on for one program scan only. There are two types of one-shots that you can
program:
Leading edge
Trailing edge
Leading Edge OneĆShot
The leading edge one-shot programming technique sets a bit for one scan when
its input condition has made a false-to-true transition. The false-to-true
transition represents the leading edge of the input pulse. The programming for
a leading edge one-shot is shown in Figure 11.4.
Objectives
OneĆShot