Programming Logic Controllers

Creating Relay Logic Diagrams - Chapter 5

Relay Logic Diagrams

• Also called • Elementary diagrams• Line diagrams• Relay ladder logic (RLL)

• Show logical relationships between devices

• Symbols for input and output devices• 1st step in creating a PLC program

Relay Logic Diagrams

• Relay logic diagrams are converted to PLC ladder logic diagrams used to program the PLC

• Two rails • (usually 120VAC, 24VDC, or 12VDC)• L1 = supply• L2 = common

• Rungs• Row in a ladder logic diagram• Diagrams can have many rows

9 Rules for Drawing Relay Logic Diagrams

1. Input devices are placed near the left corner of the rung.

2. One (and only one) output is placed near the right corner of the rung.

3. Input devices can be connected in series, parallel, or a combination of series and parallel.

9 Rules for Drawing Relay Logic Diagrams

4. Output devices cannot be connected in series. Only parallel output devices can be placed on a rung.

5. Input devices can be represented multiple times but output devices can appear only once.

6. All input and output devices must be placed horizontally.

9 Rules for Drawing Relay Logic Diagrams

7. Current in the relay logic diagram must flow from left to right.

8. Diagrams must be numbered and commented correctly. Rung numbers placed on the left of each rung. Instruction symbols and comments are placed above each instruction. Rung comments are placed on the right side of each rung.

9. The location of each contact associated with a coil can be recorded by the right hand rail near the coil.

Creating Relay Logic Diagrams for Industrial Control Circuits

1. Define the problemA description of operation

2. Draw the relay logic diagramMake sure the diagram matches the problem description of operation