Page 104 - Programmable Logic Controllers, Fifth Edition

Symbol
Output energize

15 14 13 12 11 10 98 76 54 32 1 0
Output 1
data

ON
I:1/1 I:1/4 O:2/1 O:2/1
Program

Output
module
15 14 13 12 11 10 98 76 54 32 1 0
Input 1 1
data

I:1/1
I:1/4

Input
module
Figure 5-23 Output Energize (OTE) instruction.

Sometimes beginner programmers who are used to the concept. A better approach is to separate the action
thinking in terms of hardwired relay control circuits of the field device from the action of the PLC bits as
tend to use the same type of contact (NO or NC) in the illustrated in Figure 5-24. A signal present makes the
ladder logic program that corresponds to the type of NO bit (1) true; a signal absent makes the NO bit (0)
field switch wired to the discrete input. While this is false. The reverse is true for an NC bit. A signal present
true in many instances, it is not the best way to think of makes the NC bit (1) false; a signal absent makes the
NC bit (0) true.
Input Input The main function of the ladder logic diagram pro-
module module gram is to control outputs based on input conditions, as
illustrated in Figure 5-25. This control is accomplished
A Bit A Bit through the use of what is referred to as a ladder rung. In
status status general, a rung consists of a set of input conditions, rep-
0 1 resented by contact instructions, and an output instruction
at the end of the rung, represented by the coil symbol.

Ladder logic program Ladder logic program Inputs Outputs

FALSE TRUE I/1 I/2 I/3 O/1
A OUTPUT A OUTPUT
OFF ON
Rung 0 O/1
A A
ON OFF
TRUE FALSE I/2 I/4 O/2
Button not actuated Button actuated Rung 1

Figure 5-24 Separating the action of the field device and
PLC bit. Figure 5-25 Ladder logic diagram rungs.

Basics of PLC Programming Chapter 5 85

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