Copyright Goodheart-Willcox Co., Inc. 94 Programmable Logic Controllers: Hardware and Programming SW TSW L1 L2 1 2 3 M1 M1 M1 Green pilot light Red pilot light Motor #1 1,2, 3 Goodheart-Willcox Publisher Figure 6-8. Placing comments on the ladder diagram. 6.2 Programming Ladder Logic Diagrams with Normally Open and Normally Closed Switches In Chapter 5—Creating Relay Logic Diagrams, you studied how to create the relay logic diagram. The next step after constructing the relay logic diagram for a control system is to assign addresses to each input and output device. Next, you connect the input/output devices to the PLC ports. Finally, you create the PLC ladder diagram. Note that the PLC must be in the offline mode while the ladder logic diagram is created. Figure 6-9 displays a relay logic diagram showing that when the normally open switch SW1 is closed, red pilot light R turns on. When both normally open switch SW1 and normally open limit switch LS1 are closed, both the motor M1 and red pilot light R turn on. Figure 6-10 displays the input/output connections of an Allen-Bradley fixed SLC 500 PLC device for the relay logic diagram in Figure 6-9. Notice that the Allen-Bradley fixed SLC 500 PLCs have twelve inputs and eight outputs on only one fixed rack (rack or module zero). Table 6-1 illustrates the input/ output ports assignments. Stop SW1 LS1 M1 L1 L2 R Goodheart-Willcox Publisher Figure 6-9. Relay logic diagram. Input Ports Output Ports NC stop pushbutton = I:0/0 Motor #1 = O:0/0 NO start switch = I:0/1 Red light = O:0/1 Limit switch = I:0/2 Goodheart-Willcox Publisher Table 6-1. Input/output address assignments for Figure 6-9.