Programmable Logic Controllers: Hardware and Programming, 4th Edition Page 93 (107 of 446)

Copyright Goodheart-Willcox Co., Inc. Chapter 6 PLC Programming 93 I:0/0 :0/0 I:0/1 I:0/2 L1 L2 O:0/0 :0/0 I:0/0 I:0/3 I 3 I:0/5 I:0/4 I:0/ O:0/1 Goodheart-Willcox Publisher Figure 6-4. Incorrect ladder diagram. All contacts must be placed horizontally. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 I:0/0 I:0/3 I:0/5 I:0/4 O:0/1 Goodheart-Willcox Publisher Figure 6-5. Corrected version of the diagram from Figure 6-4. I:0/0 I I:0/1 L1 L2 O:0/1 O:0/0 0 Goodheart-Willcox Publisher Figure 6-6. Incorrect ladder diagram. Only one output can be connected in series with a group of contacts. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 I:0/0 I:0/3 I:0/5 I:0/3 I:0/4 O:0/1 Correct Incorrect Goodheart-Willcox Publisher Figure 6-7. The flow in ladder diagrams is from left to right. • Rule 6. Place the rung numbers on the left side of each rung. The instruction address and device descriptions are placed above the instruction. Rung comments are placed on the top of each rung and describe the major function of the following rung or rungs. You can also specify the location of each contact associated with a coil by placing next to the coil the number of the rung on which the associated contact is shown. For example in Figure 6-8, the numbers 1, 2, and 3 are placed next to the coil for motor #1 to indicate that rungs 1 and 3 contain the related normally open contacts and rung 2 contains the related normally closed contacts.

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Copyright Goodheart-Willcox Co., Inc. Chapter 6 PLC Programming 93 I:0/0 :0/0 I:0/1 I:0/2 L1 L2 O:0/0 :0/0 I:0/0 I:0/3 I 3 I:0/5 I:0/4 I:0/ O:0/1 Goodheart-Willcox Publisher Figure 6-4. Incorrect ladder diagram. All contacts must be placed horizontally. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 I:0/0 I:0/3 I:0/5 I:0/4 O:0/1 Goodheart-Willcox Publisher Figure 6-5. Corrected version of the diagram from Figure 6-4. I:0/0 I I:0/1 L1 L2 O:0/1 O:0/0 0 Goodheart-Willcox Publisher Figure 6-6. Incorrect ladder diagram. Only one output can be connected in series with a group of contacts. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 I:0/0 I:0/3 I:0/5 I:0/3 I:0/4 O:0/1 Correct Incorrect Goodheart-Willcox Publisher Figure 6-7. The flow in ladder diagrams is from left to right. • Rule 6. Place the rung numbers on the left side of each rung. The instruction address and device descriptions are placed above the instruction. Rung comments are placed on the top of each rung and describe the major function of the following rung or rungs. You can also specify the location of each contact associated with a coil by placing next to the coil the number of the rung on which the associated contact is shown. For example in Figure 6-8, the numbers 1, 2, and 3 are placed next to the coil for motor #1 to indicate that rungs 1 and 3 contain the related normally open contacts and rung 2 contains the related normally closed contacts.

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Copyright Goodheart-Willcox Co., Inc. 92 Programmable Logic Controllers: Hardware and Programming 6.1 Creating PLC Ladder Logic Diagrams from Relay Logic Diagrams Figure 6-1 shows a simple relay logic diagram. In relay logic diagrams, input and output device symbols are used to create the diagram. In Chapter 5—Creating Relay Logic Diagrams, you learned the nine rules accepted as standard for creating relay logic diagrams. Figure 6-2 shows the PLC ladder logic diagram for the relay logic diagram displayed in Figure 6-1. Notice that input and output port addresses have been assigned to each input and output device. In Figure 6-2, SW1 and SW2 switches and limit switch LS1 are connected to input terminals I0, I1, and I2 of the input module zero (slot zero). They are therefore labeled I:0/0, I:0/1, and I:0/2 in the ladder diagram. When all three switches are closed, the control relay coil is energized. The control relay coil is connected to output port zero of the output module zero (O:0/0). To convert a relay logic diagram to a ladder logic diagram, programmers follow six standard rules: • Rule 1. Place a contact in the upper-left corner of the ladder logic diagram. (See Figure 6-2.) • Rule 2. Place the coil at the right end of the rung. Compare the correct diagram in Figure 6-2 with the incorrect diagram in Figure 6-3. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 Figure 6-2. The PLC ladder diagram. I:0/0 I:0/1 I:0/2 L1 L2 O:0/0 /0 Goodheart-Willcox Publisher Figure 6-3. Incorrect ladder diagram. Coils must appear at the end of the rung. • Rule 3. All contacts must be placed horizontally. No vertical contacts are allowed. Compare the incorrect diagram in Figure 6-4 with the correct diagram in Figure 6-5. • Rule 4. Outputs cannot be connected in series with other outputs. They can only be connected in parallel. See the incorrect ladder diagram in Figure 6-6. • Rule 5. Program execution flow must be from left to right and from top to bottom. (See Figure 6-7.) SW1 SW2 LS1 M1 L1 L2 Goodheart-Willcox Publisher Figure 6-1. A simple relay logic diagram.

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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.

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