Page 204 - Programmable Logic Controllers, Fifth Edition - Mobile version

9.1 Program Control
SBR (Subroutine)—Identifies the subroutine program.
TND (Temporary End)—Makes a temporary end
Several output-type instructions, which are often referred
to as override instructions, provide a means of execut- that halts program execution.
ing sections of the control logic if certain conditions are MCR (Master Control Reset)—Clears all set non-
met. These program control instructions allow for greater retentive output rungs between the paired MCR
program flexibility and greater efficiency in the program instructions.
scan. Portions of the program not being utilized at any SUS (Suspend)—Identifies conditions for debugging
particular time can be jumped over, and outputs in specific and system troubleshooting.
zones in the program can be left in their desired states.
Program control instructions are used to enable or dis- 9.2 Master Control Reset Instruction
able a block of logic program or to move execution of a
program from one place to another place. Figure 9-1 shows Hardwired master control relays are used in relay con-
the Program Control menu tab for the Allen- Bradley trol circuitry to provide input/output power shutdown of
SLC 500 PLC and its associated RSLogix software. The an entire circuit. Figure 9-2 shows a typical hardwired
program control commands can be summarized as follows: master control relay circuit. In this circuit, unless the
master control relay coil is energized, there is no power
JMP (Jump to Label)—Jump forward/backward to a flow to the load side of the MCR contacts.
corresponding label instruction. The equivalent PLC instruction to a Master Control
LBL (Label)—Specifies label location. Relay is the Master Control Reset (MCR) instruction.
JSR (Jump to Subroutine)—Jump to a designated This instruction functions in a similar manner to the
subroutine instruction. hardwired master control relay; that is, when the instruc-
RET (Return from Subroutine)—Exits current sub- tion is true, the circuit functions normally, and when the
routine and returns to previous condition. instruction is false, nonretentive outputs are switched off.

JMP LBL JSR RET SBR TND MCR SUS

Program Control Ascii Control Ascii String Micro

Figure 9-1 Program Control menu tab.

L1 L2
Master start
Master stop
MCR
MCR

MCR MCR

CR

CR4 OL
M1
CR1
CR2 M1 OL
M2

Figure 9-2 Hardwired master control relay.
Source: This material and associated copyrights are proprietary to, and used with the permission of
Schneider Electric.

Program Control Instructions Chapter 9 185

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