Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicL1 L2 LS14 PS1 CR10 L1 L2LS1410 11 10010PS1*LS151112*Wired NCProgrammed NOCR10100CR10100LS1512SOL750Figure 12. PLC ladder diagram of the circuit in Figure 11.50SOL7I/O AddressDevice10LS1411PS112LS1550SOL7100CR10TypeInputInputInputOutputInternalTable 7. I/O address assignment table.Master Control Relays. Another circuit the programmer should be awareof is a master control relay (MCR). In electromechanical circuit diagrams,an MCR coil controls several rungs in a circuit by switching ON or OFFthe power to those rungs. In a hardwired circuit, there is no definite end to anMCR except when the circuit is followed all the way through. For example,in Figure13, the MCR output in line 1 controls the power to the hardwired123L1PS1CR1MCRLS1MCRPL1L2MCR controlspower to circuitsbelow until theend of thehardwired circuitPower toother circuitsnot controlledby MCR4HardwiredCircuits50 HardwiredCircuits51LS100TS20CR100Last hardwiredcircuitFigure 13. Electromechanical relay circuit with a master control relay.Industrial Text & Video Company www.industrialtext.com 1-800-752-839818
Introduction to PLC Programming and Implementation—from relay logic to PLC logicelements from line 3, where the MCR contact is located, to the last elementin line 51. If the master control relay is ON, power will flow to these rungs(lines 4 through 51). If the master control relay is OFF, power will not flowand these devices will not implement the control action. This configurationis equivalent to a hardwired subprogram or subroutine—if the MCR is ON,the rungs are executed; if it is OFF, the rungs are not executed. At line 2in the circuit, power branches to other circuits that are not affected by the MCR’saction. These circuits are the regular hardwired program.During the translation from a hardwired ladder circuit to PLC symbology,the programmer must place an END MCR instruction after the last rung theMCR should control. Figure14 illustrates the placement of the MCR instructionfor the circuit in Figure 13. To provide proper fencing for the program’sMCR control section, internal output coil 1000, labeled CR1 (line 1 of PLCprogram), was inserted so that PL1 would not be inside the fenced MCRarea. This is the way the hardwired circuit operates. The END1 instructionL1 L2 PS1 LS1 CR1 L1 L2PS1010 10 11 Int 1000LS10112000PL1040040PL1CR11000MCR1TranslatedLogicFenced byMCR1TranslatedLogicLS100102LS100102TS20103Int2000TS20 103END1Rest of programfrom line 2 inhardwired circuitFigure 14. PLC ladder diagram with MCR fence.Industrial Text & Video Company www.industrialtext.com 1-800-752-839819
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Programmable Controllers: Theory and Implementation

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