Page 26 - Programmable Logic Controllers, Fifth Edition

Input module Output module
0 4 8 12 24 VDC 0 4 8 12 240 VAC
1 5 9 13 input 1 5 9 13 output
2 6 10 14 module Field device 2 6 10 14 module
3 7 11 15 power supply 3 7 11 15
L2 L1
240 VAC
IN 0 VAC
IN 1 OUT 0
IN 2 M OUT 1
IN 3 OUT 2
IN 4 OUT 3
IN 5 OUT 4
IN 6 OUT 5
IN 7 OUT 6
IN 8 OUT 7
IN 9 OUT 8
IN 10 R OUT 9
IN 11 OUT 10
IN 12 OUT 11
IN 13 OUT 12
IN 14 Y OUT 13
IN 15 OUT 14
DC OUT 15
+ 24 VDC – COM DC AC
Field device COM COM
power supply
Figure 1-14 Typical PLC input/output (I/O) system connections.

are hardwired to the input terminals. Output devices such modifying programs, and transferring programs to mul-
as small motors, motor starters, solenoid valves, and in- tiple machines.
dicator lights are hardwired to the output terminals. To A personal computer (PC) is the most commonly used
electrically isolate the internal components from the input programming device. Most brands of PLCs have soft-
and output terminals, PLCs commonly employ an optical ware available so that a PC can be used as the program-
isolator, which uses light to couple the circuits together. ming device. This software allows users to create, edit,
The external devices are also referred to as “field” or document, store, and troubleshoot ladder logic programs
“real-world” inputs and outputs. The terms field or real (Figure 1-15). The computer monitor is able to display
world are used to distinguish actual external devices that more logic on the screen than can hand-held types, thus
exist and must be physically wired from the internal user simplifying the interpretation of the program. The per-
program that duplicates the function of relays, timers, and sonal computer communicates with the PLC processor
counters. via a serial or parallel data communications link, or
A programming device is used to enter the desired Ethernet. If the programming unit is not in use, it may be
program into the memory of the processor. The program unplugged and removed. Removing the programming
can be entered using relay ladder logic, which is one of unit will not affect the operation of the user program.
the most popular programming languages. Instead of A program is a user-developed series of instructions
words, ladder logic programming language uses graphic that directs the PLC to execute actions. A programming
symbols that show their intended outcome. A program in language provides rules for combining the instructions
ladder logic is similar to a schematic for a relay control so that they produce the desired actions. Relay ladder
circuit. It is a special language written to make it easy logic (RLL) is the standard programming language used
for people familiar with relay logic control to program with PLCs. Its origin is based on electromechanical
the PLC. Hand-held programming devices are sometimes relay control. The relay ladder logic program graphically
used to program small PLCs because they are inexpen- represents rungs of contacts, coils, and special instruc-
sive and easy to use. Once plugged into the PLC, they tion blocks. RLL was originally designed for easy use
can be used to enter and monitor programs. Both com- and understanding for its users and has been modified
pact hand-held units and laptop computers are frequently to keep up with the increasing demands of industry’s
used on the factory floor for troubleshooting equipment, control needs.

Programmable Logic Controllers (PLCs) Chapter 1 7

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