108 Programmable Logic Controllers: Hardware and Programming
6.3 Programming Ladder Logic Diagrams
with Normally Open and Normally Closed
Switches
In Chapter 5, you studied how to create the relay logic diagram.
The next step after constructing the relay logic diagram for a control
system is to assign addresses to each input and output device. Next,
the input/output devices are connected to the PLC ports. Then, the
PLC ladder diagram is created.
Figure 6-9 displays a relay logic diagram. This diagram states that
when the normally open switch SW1 is closed, red pilot light R turns
on. When both normally open switch SW1 and normally open limit
switch LS1 are closed, both the motor M1 and red pilot light R turn on.
In Chapter 2, you started learning about the Allen-Bradley
SLC 500 series programmable logic controllers. You will use these
PLCs to implement the ladder logic programs throughout this text.
Figure 6-10 displays the input/output connections of an Allen-Bradley
fixed SLC 500 PLC device.
Notice that the Allen-Bradley fixed SLC 500 programmable logic
controllers have twelve inputs and eight outputs on only one fixed
rack (rack or module zero). Table 6-1 illustrates the input/output ports
assignments.
When creating a new PLC file, the first step is to select the proper
PLC processor. For an Allen-Bradley fixed PLC with twelve 120 VAC
input and eight 120 VAC output, the processor type is 1747-L20A. We
will use the Rockwell RSLogix 500 software to create the PLC ladder
diagram. Figure 6-11 displays the
Select Processor Type
dialog box.
Figure 6-12 shows the PLC ladder diagram for the relay logic dia-
gram displayed in Figure 6-9. The PLC was in the offline program-
ming mode while the ladder diagram was created. Offline mode is the
mode where the PLC ladder logic diagram can be created.
Stop SW1 LS1
M1
L1 L2
R
Figure 6-9. Relay logic diagram.