Inputs Ladder logic program Output Position_2_Switch <Local:1:I.Data.2> Internal_Relay Position_1_Switch Room_Light Position_1_Switch <Local:1:I.Data.1> Position_3_Switch <Local:1:I.Data.3> Position_3_Switch <Local:1:I.Data.3> Internal_Relay Monitor Tags Window Position_1_Switch <Local:1:I.Data.1> Position_2_Switch <Local:1:I.Data.2> Internal_Relay Name Value Force Mask Room_Light <Local:2:O.Data.5> Style Data Type Internal_Relay 0 Decimal BOOL Position_1_Sw... 0 Decimal BOOL Position_2_Sw... 0 Decimal Decimal BOOL Position_3_Sw... 0 BOOL Room_Light 0 Decimal BOOL L1 Position_2_Switch Position_3_Switch Figure 15-38 Internal relay to implement on/off control of a room light from three different entrances. Figure 15-38 shows a ControlLogix program that uses an internal relay to implement on/off control of a room light from three different entrances or positions. Three single pole switches are used for inputs in place of the two 3-way and one 4-way switches normally required for an equivalent hardwired control circuit. The operation of the program can be summarized as follows: • An internal relay is used to execute the logic of the circuit without having to use a real-world output. • The status value stored in memory for all tags, when all input switches are open, is 0 and so the room light will be off. • Closing Position_1_Switch changes the status of its XIC instruction from false to true thereby establishing logic continuity for Rung 1. • As a result, the status of the internal relay coil and its XIC contact change from false to true. • This establishes logic continuity for Rung 2 and switches the room light on. • A change in the state of any of input switches will change the current state of the light. Internal relay output coils are used in ladder logic programs, but are not accessible at I/O racks. They are used where L L2 an output instruction is required in a program, but no physical connection to a real world device is needed. A hardwired pilot light motor control circuit is shown in Figure 15-39. The operation of the circuit can be summarized as follows: • The Stop/Start pushbutton station controls relay coil CR. • When CR is de-energized, the green standby pilot light is ON, the red run pilot is OFF, and the motor is not operating. L1 L2 Stop Start Control relay CR Standby PL CR-1 CR-2 CR-3 G Run PL R CR-4 Motor M Figure 15-39 Hardwired pilot light motor control circuit Bit-Level Programming Part 2 351
Petruzella_ProgrammableLogicControllers__5e
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