104 Programmable Logic Controllers: Hardware and Programming
6.1 Introduction
A relay logic diagram shows the logical relationships between
devices. A ladder logic diagram is the program loaded into the pro-
grammable logic controller. In this chapter, you will learn the correct
method of drawing ladder logic diagrams. The routine for converting
the relay logic diagrams to PLC ladder diagrams will be illustrated.
You will study how to program the Allen-Bradley Small Logic
Controller (SLC 500) series programmable logic controllers. The
Rockwell RSLogix 500 software will be used to program the SLC 500
series PLCs.
You will also learn how to troubleshoot a PLC system using force
instruction, create reports, and use the PLC utility software com-
mands. Throughout the chapter, there are several PLC examples. These
examples illustrate all the steps required to implement the objectives.
6.2 Creating PLC Ladder Logic Diagrams
from Relay Logic Diagrams
Figure 6-1 shows a simple relay logic diagram. In relay logic dia-
grams, input and output device symbols are used to create the dia-
gram. In Chapter 5, you learned that there are nine rules accepted as
standard for creating relay logic diagrams.
Figure 6-2 shows the PLC ladder logic diagram for the relay logic
diagram displayed in Figure 6-1. Notice that input and output port
addresses have been assigned to each input and output device. In relay
logic and ladder logic diagrams, each row is called a rung. Diagrams in
Figure 6-1 and Figure 6-2 each have one rung.
In Figure 6-2, SW1 and SW2 switches and limit switch LS1 are
connected to input terminals I0, I1, and I2 of the input module zero
(slot zero). They are therefore labeled I:O/0, I:O/1, and I:O/2 in the
ladder diagram. When all three switches are closed, the control relay
coil is energized. The control relay coil is connected to output port one
of the output module zero (O:0/0).
When converting relay logic diagrams to the ladder logic dia-
grams, one must follow six rules accepted as standard. These rules are
explained in the following:
Rule 1. Place a contact in the upper-left corner of the ladder logic
diagram. (See Figure 6-2).
Rule 2. Place the coil at the end of the rung. Compare the correct
diagram in Figure 6-2 with the incorrect diagram in Figure 6-3.