Page 86 - Programmable Logic Controllers, Fifth Edition - Mobile version

A L1 START L2
A + B STOP OL
B M
C M
C Y
Output
D G
D M
Inputs

Figure 4-20 Determining the Boolean equation for a logic circuit. R
M
Rail
A Rung
A AB
1
Figure 4-22 Motor stop/start relay ladder schematic.
Inputs Y relays and relay ladder schematics. Relay ladder sche-
Output
matics are universally used and understood in industry.
2 Figure 4-22 shows a typical relay ladder schematic of a
B
B AB
motor stop/start control station with pilot lights. The control
scheme is drawn between two vertical supply lines. All the
Figure 4-21 Determining the Boolean equation for a logic circuit. components are placed between these two lines, called rails
or legs, connecting the two power lines with what look like
The Boolean equation for the logic circuit of rungs of a ladder—thus the name, relay ladder schematic.
Figure 4-21 is determined as follows: Hardwired logic is fixed; it is changeable only by
–
• The output of AND gate 1 is AB altering the way devices are electrically interconnected. In
–
• The output of AND gate 2 is AB contrast, programmable control is based on the basic logic
• Based on the combination of inputs applied to the functions, which are programmable and easily changed.
These functions (AND, OR, NOT) are used either singly
OR gate the Boolean equation for the circuit is or in combinations to form instructions that will determine
–
–
Y = AB + AB
if a device is to be switched on or off. The form in which
4.6 Hardwired Logic versus these instructions are implemented to convey commands
Programmed Logic to the PLC is called the language. The most common
PLC language is ladder logic. Figure 4-23 shows a typi-
The term hardwired logic refers to logic control functions cal ladder logic program for the motor start/stop circuit.
that are determined by the way devices are electrically in- The instructions used are the relay equivalent of normally
terconnected. Hardwired logic can be implemented using open (NO) and normally closed (NC) contacts and coils.

Relay coil
equivalent
Input STOP START OL M Output
L1 module module L2
Rung 1
STOP M
M
M G
Rung 2
START
Relay contact
equivalent G

M R
OL
Rung 3
R
PROGRAM
Figure 4-23 Motor stop/start ladder logic program.

Fundamentals of Logic Chapter 4 67

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