Programmable Controllers: Theory and Implementation

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