Page 82 - Programmable Logic Controllers, Fifth Edition

Hardwired circuit
Truth table Logic representation
SW-A SW-B
SW- A SW- B Light SW-A Light
Open (0) Open (0) O (0)
Light Open (0) Closed (1) O (0)
Closed (1) Open (0) O (0)
Closed (1) Closed (1) On (1) SW-B

Figure 4-5 AND logic gate operates similarly to control devices connected in series.

illustrates the four possible combinations of inputs for a 0
2-input AND gate. The basic rules that apply to an AND A Output (Y ) = 0
gate are: B 0

• If all inputs are 1, the output will be 1. 0
• If any input is 0, the output will be 0. A Truth table
1 Output (Y ) = 1 Inputs Output
The AND logic gate operates similarly to control B A B Y
devices connected in series, as illustrated in Figure 4-5. 0 0 0
The light will be on only when both switch A and switch A 1 0 1 0 1 1 1
B are closed. 0 Output (Y ) = 1 1 1 1
B
The OR Function
1
The symbol drawn in Figure 4-6 is that of an OR gate. An A
OR gate can have any number of inputs but only one out- B 1 Output (Y ) = 1
put. The OR gate output is 1 if one or more inputs are 1.
The truth table in Figure 4-6 shows the resulting output Y Figure 4-7 OR logic gate digital signal states.
from each possible input combination.

Figure 4-7 illustrates the four possible combinations of
inputs for a 2-input OR gate. The basic rules that apply to
OR truth table
Inputs Output an OR gate are:
A
Inputs Y A B Y • If one or more inputs are 1, the output is 1.
B Output 0 0 0 • If all inputs are 0, the output will be 0.
0 1 1
Two-input OR 1 0 1 The OR logic gate operates similarly to control devices
gate symbol 1 1 1
connected in parallel, as illustrated in Figure 4-8. The
Figure 4-6 OR gate. light will be on if switch A or switch B or both are closed.

Hardwired circuit

SW-A Truth table
Logic representation
SW- A SW- B Light SW-A Light
SW-B Open (0) Open (0) O (0)
Open (0) Closed (1) On (1)
Light Closed (1) Open (0) On (1)
Closed (1) Closed (1) On (1) SW-B

Figure 4-8 OR logic gate operates similarly to control devices connected in parallel.

Fundamentals of Logic Chapter 4 63

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