1. Programming Logic Controllers
Creating Relay Logic Diagrams - Chapter 5

2. 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

3. 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

4. 9 Rules for Drawing Relay Logic Diagrams
Input devices are placed near the left corner of the rung.
One (and only one) output is placed near the right corner of the rung.
Input devices can be connected in series, parallel, or a combination of series and parallel.

5. 9 Rules for Drawing Relay Logic Diagrams
Output devices cannot be connected in series. Only parallel output devices can be placed on a rung.
Input devices can be represented multiple times but output devices can appear only once.
All input and output devices must be placed horizontally.

6. 9 Rules for Drawing Relay Logic Diagrams
Current in the relay logic diagram must flow from left to right.
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.
The location of each contact associated with a coil can be recorded by the right hand rail near the coil.

7. Creating Relay Logic Diagrams for Industrial Control Circuits
Define the problem
A description of operation
Draw the relay logic diagram
Make sure the diagram matches the problem description of operation