Chapter 6 PLC Programming 115
Immediate input with
mask (IIM):
An instruction placed
in the middle of a
program that enables the
critical input port to be
read twice within a six
millisecond period.
Immediate output with
mask (IOM):
An instruction placed in
the middle of a program
that enables the critical
output port to be read
twice within a six
millisecond period.
Project tree area:
Area in RSLogix 500
software that holds the
PLC data files and other
PLC project files.
Main program file:
File that contains the
main PLC ladder logic
diagram. In Allen-
Bradley SLC 500 series
PLC systems, a main file
is in file two (LAD 2).
PLC project:
Project that contains the
PLC ladder diagrams
and the PLC data files.
Subroutine
program files:
Files called from the
main PLC program file.
Subroutine program files
can be viewed in the
Program Files area.
When a rung is scanned, its input and output addresses are
updated. However, because the scanning takes place from left to right
across each rung, and from the top to bottom rungs, sometimes spe-
cific inputs or outputs may not be updated in time.
For example, assume the scan time for a ladder program is six milli-
seconds, but the status of an input port changes every three milliseconds.
This results in missing the status of that input port every other time.
In order to correct this problem, an instruction called immediate
input with mask (IIM) is placed in the middle of the program. The
critical input port is then read twice within a six milliseconds period.
There is a similar instruction called immediate output with mask
(IOM) that can be placed for the output port(s).
6.6 PLC Program and Data Files
A programmable logic controller project consists of the ladder
logic diagram file and its associated data files. These files are shown in
Figure 6-20 in the project tree area of the RSLogix 500 software pro-
gram. Notice that the project area is divided into two areas:
Program
Files
and
Data
Files.
There are at least three program file icons in the
Program Files
area.
The
SYS0
program file is used by the PLC processor to keep track of its
internal register settings. The
SYS1
program file is reserved. Therefore,
these two system files are not available to the RSLogix 500 software user.
PLC programmers have access to at least one program file called
the main program file. The main program file (LAD2) holds the main
PLC ladder logic diagram.
In a PLC project, there may be more than one program file. This
means that there might be several subroutine program files in addi-
tion to the main program file. These subroutine program files are
called on or accessed from the main program file. Subroutine files and
Output scan
Input scan
Program scan
Figure 6-19. PLC scan cycle.