Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicends the MCR fence. The instructions corresponding to the hardwiredcircuits that branch from line 2 in the electromechanical diagram of Figure 13are located after the END1 instruction. Figure15 illustrates a partial ladderrung of a more elaborate circuit with this type of MCR condition. Thecorresponding PLC program should have an END MCR after the rungcontaining the PL3 output.123Set Up/RunRunCR1CR2UpLS1CR1M1CR1CR2OLs4567EnableUpCR1MCRPL2CR3MasterControlRelayMaster ONUp89MCRLS2CR4CR3TDR1SOL1Sol Up10CR3TDR1CR3SOL2Sol Dn11LS3PL3Dn ON121314FeedLS4CR4CR4CR4PL4SOL3Set UpSet Up ON15Feed Sol16CR3CR5SOL4Fast Sol17CR41819CR1 LS5 TDR1CR25 secondsFigure 15. Electromechanical relay circuit with an MCR.Industrial Text & Video Company www.industrialtext.com 1-800-752-839820
Introduction to PLC Programming and Implementation—from relay logic to PLC logicEXAMPLE 3Highlight the sections of the circuit in Figure 15 that will be under thecontrol of a PLC MCR. What additional measures must be taken toinclude or bypass other hardwired circuits within the MCR fence?SOLUTIONFigure 16 highlights the circuits that must be fenced under the MCRinstruction. Note that solenoid SOL1 and part of its driving logic are notincluded in the MCR fencing because SOL1, CR3, and TDR1 can alsobe turned ON by logic prior to the MCR fence (see Figure 17). For theMCR fence to be properly programmed, the PLC program must123Set Up/RunRunCR1CR2UpLS1CR1M1CR1CR2OLs4567EnableUpCR1MCRPL2CR3MasterControlRelayMaster ONUp89MCRLS2CR4CR3TDR1SOL1Sol Up10111213141516171819FeedLS4CR3CR4CR4CR5CR3LS3TDR1CR3CR4CR4CR1 LS5 TDR1CR2SOL2PL3PL4SOL3SOL45 secondsSol DnDn ONSet UpSet Up ONFeed SolFast SolFigure 16. MCR-controlled program elements.Industrial Text & Video Company www.industrialtext.com 1-800-752-839821
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Programmable Controllers: Theory and Implementation

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