GE GFK-0804B PLC Programs

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1-3GFK-0804B Chapter 1 Programming for the Micro PLC
PLC Programs
A typical PLC application program is created in a format called ladder logic.
1
2
3
Input 1
Input 2
Input 3
Input 4 Input 5 Output 3
Output 2
Output 1
46101
Each symbol in the ladder logic represents a type of input, output, or other program
action. There are many types of symbols. The three symbols shown above are:
normally–open
contact
normally–open
contact normally–closed contact coil
Inputs Output 46102
In the ladder logic, each line or group of lines that ends in an action being performed, such
as an output being sent, is called a rung. In the example above, there are three rungs.
Power Flow in a Program
The PLC executes the logic in the ladder from top to bottom, one rung at a time. Within
each rung, the execution flows from left to right. This movement of program execution
through the ladder can also be thought of as power flow. In the example:
Rung 1: Input 1 represents a switch. It is shown in the ladder logic program as a
“normally–open” contact. When the switch is turned on, the input 1 con-
tact closes and power flows across rung 1 to the coil labelled Output 1.
Rung 2: Rung 2 begins at the left side with two lines of logic that lead to the same
output on the right. In this type of rung, which can have several lines
beginning on the left, the output is ON if any of the input lines can be
completed. In this rung, if either Input 2 or Input 3 is closed, Output 2 is
turned ON.
Rung 3: Rung 3 illustrates the use of multiple inputs in the same line of logic. All
of these inputs must be completed for the output to be ON. In this exam-
ple, Input 4 must be closed (active), and Input 5 must be closed (inactive)
for Output 3 to be set to ON.