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Petruzella_ProgrammableLogicControllers__5e

Initially, all program development can proceed with just the tag names and data types assigned. Using tag aliases, programmers can write code independent of electrical connection assignments. At a later date, input and output field devices are easily matched to the pin numbers on the respective module they are connected to. Adding Ladder Logic to the Main Routine Figure 15-31 shows the diagram for a hardwired contactor operated motor start/stop control circuit. The normally open start button is momentarily closed to energize the contactor coil and close its main contacts to start the motor. The seal-in auxiliary contact of the contactor is connected in parallel with the start button to keep the starter coil energized when the start button is released. The normally closed stop button is momentarily opened to de-energize the contactor coil and stop the motor. Figure 15-32 shows the ladder logic program for the motor start/stop control circuit and the RSLogix 5000 toolbar used to create it. Free form editing found in RSLogix 5000 helps speed development in that you do not have to assign addresses to instructions before adding more Rung Branch Bit element toolbar XIC XIO OTE OTL OTU L U Ladder logic program ONS OSR Bit Stop Terminal Terminal Contactor 4 4 3 Start ? ? ? ? M + + Common Common Field device power DC INPUT DC OUTPUT ST 0 1 2 3 4 5 6 7 ST 0 1 2 3 4 5 6 7 Input module Slot 1 348 Part 2  Bit-Level Programming L1 L2 Motor start Contactor auxiliary contact Motor run M Contactor coil Motor stop Figure 15-31  Hardwired motor start/stop control circuit. instructions. In this example we have chosen to use question marks ? in place of tag names and assign the tags later. Field device wiring for the two pushbutton inputs and the single contactor coil output are as illustrated. The stop button is connected to terminal 4 and the start button to terminal 3 of the DC input module located in slot 1 of the rack. The contactor coil is connected to terminal 4 of the DC output module located in slot 2 of the rack. Both the start and stop buttons are examined for a closed condition (XIC) because both buttons must be closed to cause the motor starter to operate. Field device power Output module Slot 2 Figure 15-32  Programmed motor start/stop control circuit.


Petruzella_ProgrammableLogicControllers__5e
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