Page 87 - Programmable Logic Controllers, Fifth Edition

PLC contact symbolism is a simple way of expressing
Because the PLC uses ladder logic diagrams, the con-
the control logic in terms of symbols. These symbols are version from any existing relay logic to programmed
basically the same as those used for representing hard- logic is simplified. Each rung is a combination of input
wired relay control circuits. A rung is the contact sym- conditions (symbols) connected from left to right, with
bolism required to control an output. Some PLCs allow the symbol that represents the output at the far right.
a rung to have multiple outputs while others allow only The symbols that represent the inputs are connected in
one output per rung. A complete ladder logic program then series, parallel, or some combination of the two to obtain
consists of several rungs, each of which controls an output. the desired logic. The following group of examples
In programmed logic all mechanical switch contacts are illustrates the relationship between the relay ladder sche-
represented by a software contact symbol and all electro- matic, the ladder logic program, and the equivalent logic
magnetic coils are represented by a software coil symbol. gate circuit.

Relay schematic Ladder logic program
Gate logic
LS1 LS2 SOL A B Y A
Y
LS1 LS2 SOL B Output
Inputs
Boolean equation: AB = Y
Example 4-1 Two limit switches connected in series and used to control a solenoid valve.

Relay schematic Ladder logic program
Gate logic
LS1 SOL A Y
A
Y
LS1 SOL
LS2 B
B Output
Inputs
LS2
Boolean equation: A + B = Y
Example 4-2 Two limit switches connected in parallel and used to control a solenoid valve.

Relay schematic Ladder logic program Gate logic
PL
LS1 PS A C Y A
G A B
LS1 PS PL Y
LS2 B
B C Output
Inputs
LS2
Boolean equation: (A + B)C = Y
Example 4-3 Two limit switches connected in parallel with each other and in series with a pressure switch.

Relay schematic Ladder logic program Gate logic
PL A
LS1 FS1 A C Y A + B
R
LS1 FS1 PL B
Y
LS2 FS2 B D C Output
LS2 FS1 D C + D
Inputs
Boolean equation: (A + B) (C + D) = Y

Example 4-4 Two limit switches connected in parallel with each other and in series with two sets of flow
switches (that are connected in parallel with each other), and used to control a pilot light.

68 Chapter 4 Fundamentals of Logic

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