Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicinclude two internal control relays that take SOL1 out of the fence.Figure 18 illustrates the fenced circuit with the additional internals(CR1000 and CR1001). Note that the instructions in this diagram havethe same names as in the hardwired circuit. The solenoid SOL1 will beoutside of the MCR fence because it can be turned ON by either theoutside logic (highlighted section in Figure 17) or the logic inside theMCR fence (highlighted section in Figure 18).789Set Up/RunMCRUpLS2CR4CR1 CR3add CR1000SOL1CR3 TDR1UpSol Up10CR3TDR1CR3SOL2Sol Dn11LS3add CR1001PL3Dn ONFigure 17. SOL1 activated by logic outside of the MCR fence.Set Up/Run Up CR4 CR1000LogicDriving MCRMCR1Up LS2 LS1 CR1001UpCR3TDR1 CR3 SOL2LS3Fenced byMCRSOL2PL3END1CR1000CR3 TDR1 SOL1CR1001Figure 18. MCR fence.Industrial Text & Video Company www.industrialtext.com 1-800-752-839822
Introduction to PLC Programming and Implementation—from relay logic to PLC logicBidirectional Power Flow. The circuit in Figure 19 illustrates anothercondition that can cause programming problems: the possibility of bidirectionalpower flow through the normally closed CR4 contact in line 8. Tosolve the bidirectional flow problem, the programmer must know whether ornot CR4 influences the two output rungs to which it is connected. These rungsare the CR3 control relay output and the solenoid SOL1 output (rungs 7 and9, respectively). Figure 19 illustrates the two paths that can occur in thehardwired circuit. PLCs only allow forward paths; therefore, if a reverse pathis necessary for this circuit’s logic, the CR4 contact must be included in thelogic driving the CR3 output (see Figure 9b).7CR1CR3Up89MCRLS2CR4CR3TDR1SOL1Sol Up1011CR3 TDR1LS3CR3SOL2PL3Sol DnDn ON(a) Forward path7CR1CR3Up89MCRLS2CR4CR3TDR1SOL1Sol Up1011CR3 TDR1LS3CR3SOL2PL3Sol DnDn ON(b) Reverse pathFigure 19. (a) Forward and (b) reverse power flow in a hardwired circuit.Instantaneous Timer Contacts. The electromechanical circuit shown inFigure 15 specifies an instantaneous timer contact (the normally openTDR1 contact in line 10). This type of contact, however, is usually unavailablein PLCs. To implement an instantaneous timer contact (i.e., a contactIndustrial Text & Video Company www.industrialtext.com 1-800-752-839823
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Programmable Controllers: Theory and Implementation

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