Page 212 - Programmable Logic Controllers, Fifth Edition - Mobile version

Main program 9.5 Immediate Input and Immediate
file 2
Output Instructions
The PLC input scan normally records the inputs before the
program scan, and the output scan normally updates the
JSR outputs after the program scan. Immediate I/O instruc-
JUMP-TO-SUBROUTINE tions allow you to update data prior to the normal input
SBR file number 3
scan as illustrated in Figure 9-14.
Immediate I/O instructions interrupt the normal pro-
Subroutine gram scan to update the input image table file with cur-
file 3
SBR rent input data or to update an output module group with
SUBROUTINE the current output image table file data. Allen-Bradley
SLC 500 PLC’s immediate I/O instructions are called
immediate input with mask (IIM) and immediate
output with mask (IOM).
• Masking is a means of selectively screening out
data.
• Masking allows the programmer to specify which
of the 16 bits are to be copied from an input module
RET to the input image data table (or from the output
RETURN
image table to an output module).
Figure 9-12 Setting up a subroutine file. • The other bits in the input image table or output
module are not affected by these instructions.
Programming nested subroutines may cause scan time
problems because while the subroutine is being scanned, The immediate input with mask (IIM) instruction is
the main program is not. Excessive delays in scanning the shown in Figure 9-15. The IIM instruction operates on
main program may cause the outputs to operate later than the inputs assigned to a particular word of a slot. When
required. This situation may be avoided by updating criti- the IIM rung is true, the program scan is interrupted, and
cal I/O using immediate input and/or immediate output data from a specific input slot are transferred through the
instructions. mask to the input data file. These data are then available

Main program
file 2

JSR
JUMP-TO-SUBROUTINE
SBR file 3
Level 1 Level 2 Level 3
file 3 file 4 file 5
SBR SBR SBR
SUBROUTINE SUBROUTINE SUBROUTINE

JSR JSR
JUMP-TO-SUBROUTINE JUMP-TO-SUBROUTINE
SBR file 4 SBR file 5

RET RET RET
RETURN RETURN RETURN
Figure 9-13 Nested subroutines.

Program Control Instructions Chapter 9 193

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