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Unit 7 Discrete Controllers

1. Discrete Process Control
2. Ladder Logic Diagrams
3. Programmable Logic Controllers
4. Personal Computers Using Soft Logic
Discrete Process Control

Control systems that operate on parameters and variables

that change at discrete moments in time or at discrete
events, usually binary (0 or 1, off or on, open or closed,

Sensors Limit switch Contact/no contact

Photo-detector On/off
Timer On/off

Actuators Motor On/off

Valve Open/closed
Clutch Engaged/not engaged
Categories of Discrete Control

Logic control event-driven changes

Sequencing time-driven changes

Logic Control - a switching system whose output at

any moment is determined exclusively by the values of
No memory
No operating characteristics that depend on time
Also called combinational logic control
Elements of Logic Control

Logic gates:
AND output = 1 if all inputs = 1, zero otherwise
OR output = 1 if any input = 1, zero otherwise
NOT output = 1 if (single) input = 0, and vice versa
NAND combination of AND and NOT
NOR combination of OR and NOT
Boolean Algebra & Truth Tables

AND function is expressed as

Y = X1.X2

OR function is expressed as
Y = X1+X2

NOT function is expressed as

NAND and NOR Functions
Symbols for Logical Gates

Write the Boolean logic expression for the pushbutton switch

system below using the following symbols:
X1 = START, X2 = STOP, Y1 = MOTOR, and Y2 =



X2 X2 Y2= ( X1 Y1) X2

X1 X1+Y1

Truth Table

A switching system that uses internal timing devices to

determine when to initiate changes in output variables
Outputs are usually generated open loop
No feedback that control function is executed
Sequence of output signals is usually cyclical, as in a
high production work cycle
The signals occur in the same repeated pattern
within each regular cycle
Common sequencing devices:
Timer output switches on/off at preset times
Counter counts electrical pulses and stores them
Ladder Logic Diagrams

A diagram where logic elements are

displayed along horizontal rungs
1. Contacts - logical inputs (usually),
e.g., limit switches, photo-
2. Loads - outputs, e.g., motors,
lights, alarms, solenoids
3. Timers - to specify length of delay
4. Counters - to count pulses
Ladder Logic Diagram

Construct the ladder logic diagrams for (a) the NAND gate
and (b) the NOR gate.
X1 X2 C

(a) NAND C Y

If X1 or X2 remain open then C coil is unexcited and C contact remains closed,

therefore Y is on.
If X1 and X2 are closed then C coil is excited and C contact is opened and Y is

(b) NOR


If X1 and X2 remain open then C coil is unexcited and C

contact remains closed, therefore Y is on.
If X1 or X2 are closed then C coil is excited and C contact
is opened and Y is off

The production process may only be performed when the

operator activates two spring activated safety switches.
The switches have to be depressed and held closed
together by the operator using both hands.
(a) What is the truth table? (b) What is the Boolean logic
expression? (c) What is the logic network diagram (in ISO
symbol terminology)? (d) What is the ladder logic
Assume X1 and X2 are the first and second switches and
Y is the output

Create ladder logic diagram for Push Button switch


( X1 Y1) X2
Programmable Logic Controller (PLC)

A microcomputer-based controller that uses stored

instructions in programmable memory to
implement logic, sequencing, timing, counting,
and arithmetic functions through digital or analog
modules, for controlling machines and processes
Components of a PLC

Programming Processor
Processor I/O
Device && Memory
Memory Module

Advantages of PLCs Compared to
Relay Control Panels
Programming a PLC is easier than wiring a relay control
PLC can be reprogrammed
PLCs take less floor space
Greater reliability, easier maintenance
PLC can be connected to computer systems (CIM)
PLCs can perform a greater variety of control functions
Typical PLC Operating Cycle
1. Input scan inputs are read by processor and stored in
2. Program scan control program is executed
Input values stored in memory are used in the control
logic calculations to determine values of outputs
3. Output scan output values are updated to agree with
calculated values
Time to perform the three steps (scan time) varies
between 1 and 25 msec
PLC Programming

Graphical languages:
1. Ladder logic diagrams most widely used
2. Function block diagrams instructions
composed of operation blocks that transform
input signals
3. Sequential function charts series of steps and
transitions from one state to the next (Europe)
Text-based languages:
1. Instruction list - low-level computer language
2. Structured text high-level computer language
PLC Programming
Personal Computers Using Soft Logic

Sturdy enclosures for plant environment

Membrane-type keyboards
I/O cards and other hardware to connect to
machines and processes
Windows for implementing control
Soft logic software emulating PLCs
NXT Controller and Program