Page 143 - Programmable Logic Controllers, Fifth Edition - Mobile version

Ladder logic program
Internal
Inputs Start Stop relay Output
L1 I:3/0 I:3/1 B3:1/0 L2
Jog
I:3/2 B3:1/0
Start OL
O:2/2 M
I:3/0 Jog Stop M
I:3/2 I:3/1 O:2/2
Stop
I:3/1
B3:1/0

Figure 6-69 PLC program equivalent of the hardwired relay jog circuit.
6.12 Writing a Ladder Logic Program • List the sequence of operational steps in as much
Directly from a Narrative Description detail as possible.
• Write the ladder logic program to be used as a basis
In most cases, it is possible to prepare a ladder logic program for the PLC program.
directly from the narrative description of a control process. • Consider different scenarios where the process se-
Some of the steps in planning a program are as follows: quence may go astray and make adjustments as needed.
• Define the process to be controlled. • Consider the safety of operating personnel and
• Draw a sketch of the process, including all sensors make adjustments as needed.
and manual controls needed to carry out the control The following are examples of ladder logic programs
sequence. derived from narrative descriptions of control processes.

EXAMPLE 6-1

Figure 6-70 shows the sketch of a drilling process that PB1 PB2
requires the drill press to turn on only if there is a part Drill
present and the operator has one hand on each of the motor
start switches. This precaution will ensure that the opera- Switches
tor’s hands are not in the way of the drill.
The sequence of operation requires that switches
1 and 2 and the part sensor all be activated to make the
drill motor operate. Figure 6-71 shows the ladder logic
program required for the process implemented using an
SLC 500 controller.

Part sensor

Figure 6-70 Sketch of the drilling process.
Inputs Ladder logic program Output
L1 L2
PB1 Motor
PB1 PB2 Sensor contactor Motor
I:3/4 I:3/5 I:3/6 O:4/0 contactor
I:3/4
O:4/0 M
PB2

I:3/5
Sensor
I:3/6

Figure 6-71 Drilling process PLC program.

124 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

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