Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicrung. If the instantaneous TMR1 contacts are implemented using a PLC timedelaycontact, then PB1 must be pushed for the timer’s required time presetto latch the rung. This instantaneous contact will be implemented by trappingthe timer with an internal output.Tables 8 and 9 show the I/O address and internal output assignments for theelectromechanical circuit’s real I/O. Table 10 presents the register assignmenttable. Note that internals do not replace control relays CR1 and CR2since the output addresses 030 and 031 corresponding to solenoids SOL1 andSOL2 are available. Therefore, addresses 030 and 031 can replace the CR1and CR2 contacts, respectively, everywhere they occur in the program. Thenormally open contact LS1 connects limit switch LS1 to the PLC inputinterface; and the normally open LS1 reference, programmed with an examine-OFFinstruction, implements the normally closed LS1 in the program.Figure 24 illustrates the PLC program coding solution.ModuleTypeRackI/O AddressGroupTerminalInput0 0 0 PB10 0 1 PS10 0 2 FS10 0 3 TS1Input0 0 4 LS10 0 5 PS20 0 6 —0 0 7 —Output0 3 0 SOL10 3 1 SOL20 3 2 SOL30 3 3 —Table 8. I/O address assignment.DescriptionDeviceInternalTable 9. Internal address assignment.DescriptionTMR11000Used to trap TMR1C R1— Same as SOL1 (030)C R2— Same as SOL2 (031)TMR11001Timer TMR1TMR21002Timer TMR2CR31003Replace CR3RegisterDescription4000Preset timer count for 3 sec4001Accumulated count timer 10014002Preset timer count for 2 sec4003Accumulated count timer 1002Table 10. Register assignment.Industrial Text & Video Company www.industrialtext.com 1-800-752-839828
Introduction to PLC Programming and Implementation—from relay logic to PLC logicL1 L2L1 L2PB1 PS1 TMR TrapPB1000 0011000PS1FS1000001002TMR Trap1000TMR Trap1000TMRPR 400030AR 4001TB = 0.1TMR11001TS1003FS1002TS1003SOL1030030SOL1LS1004TMR11001SOL1030LS1004SOL2031031SOL2TMR11001SOL1030LS1004TMRPR 400220AR 4003TB = 0.1TMR21002PS2005TMR11001SOL1030CR31003TMR21002PS2005CR31003SOL3032SOL3032Figure 24. PLC implementation of the circuit in Figure 23.SIMPLE START/STOP MOTOR CIRCUITFigure 25 shows the wiring diagram for a three-phase motor and its correspondingthree-wire control circuit, where the auxiliary contacts of the starterseal the start push button. To convert this circuit into a PLC program, firstdetermine which control devices will be part of the PLC I/O system; these arethe circled items in Figure 26. In this circuit, the start and stop push buttons(inputs) and the starter coil (output) will be part of the PLC system. The startercoil’s auxiliary contacts will not be part of the system because an internal willbe used to seal the coil, resulting in less wiring and fewer connections.Industrial Text & Video Company www.industrialtext.com 1-800-752-839829
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