Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Chapter 2: Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL).
2.1
2.2
2.3
2.4
2.5
2.6
PLC Operation using scanning technique.
Understanding Relay Ladder Diagram (RLL).
Basic Instructions of RLL.
Motor control using PLC, two push buttons and motor starter.
Adding two indicators for the developed RLL.
PLC Programming.
2.1
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Programmable Logic Controller (PLC)
And Relay Ladder Logic(RLL)
PLC stands for programmable logic controller. PLCs are electronic control devices that are
used in variety of industries, ranging from manufacturing plants to processing plants. A PLC
is a device that was invented to replace the necessary sequential relay circuits for machine
control and to replace analog controller. The PLC works by looking at its inputs (both logic
and continues types) and depending upon their state, turning on/off its outputs. Examples:
PLCs used in automotive assembly plants, automotive parts manufacturing plants, mineral
processing plants, semi-conductor manufacturing plants, steel mills ….etc. The user enters a
program, usually via software, that gives the desired results.
The PLC has three components;
1) Central Processing Unit CPU,
2) Inputs,
3) Outputs.
We could consider the PLC to be a box of hundreds of separate relays, counters, timers and
data storage locations. Do these counters, timers, etc. really exist? No, they "physically"
don't exist but rather they are simulated and can be considered software counters, timers, etc.
The internal relays are simulated through bit locations in registers, as shown in Fig. 2.1.
Analog
Input
Networking
module
Analog
Ouput
Modem
Fig. 2.1 Internal structure of a PLC.
The functions of these modules are given as follows:

INPUT RELAYS-(contacts): These are connected to the outside world. They
physically exist and receive signals from switches, sensors, etc. Typically they are not
relays but rather they are transistors. These inputs are called discrete or logic inputs.
 INTERNAL UTILITY RELAYS-(contacts): These relays do not receive signals from
the outside world nor do they physically exist. They are simulated relays and are what
enables a PLC to eliminate external relays. There are also some special relays that are
dedicated to performing only one task. Some are always on while some are always off.
Some are on only once during power-on and are typically used for initializing data that
was stored.
2.2
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)

COUNTERS: These again do not physically exist. They are simulated counters and
they can be programmed to count pulses. Typically these counters can count up, down or
both up and down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include high-speed counters that are hardware based. We can
think of these as physically existing. Most of the time these counters can count up, down
or up/down.
 TIMERS: These also do not physically exist. They come in many varieties and
increments. The most common type is an on-delay type. Others include off-delay and
both retentive and non-retentive types. Increments vary from 1ms through 1s.
 OUTPUT RELAYS-(coils):These are connected to the outside world. They physically
exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or
triacs depending upon the model chosen.
 DATA STORAGE: Typically there are registers assigned to simply store the data.
They are usually used as temporary storage for math or data manipulation. They can also
typically be used to store data when power is removed from the PLC. Upon power-on
they will still have the same contents as before when power has been removed.
 ANALOG MODELS: This covers analog inputs and outputs. The analog models
cover reading analog signals from sensor, provides analog signal such as thermocouples,
strain gauges, thermistor, pressure sensor….etc. The analog output signals can be used to
command external controller e.g. servomotors amplifier, solenoid amplifier …etc.
 USER INTERFACE INPUT: Which contains extra push-bottoms that can be
configured by the user to set/reset logic output devices e.g. relays outputs, or can be used
as a storage of messages that can be displayed on liquid crystal display. Furthermore,
some of these interfaces has led which can be configured by the user.
 NETWORKING MODULES: Larger PLCs could have serial port that can be used for
networking a multiple of PLCs that are to be programmed from one main computer or
sending/receiving data between PLC network. Furthermore, some of the PLCs are
equipped with remote control module (modem) to program the PLC from long distance
computer.
The CPU of the PLC contains a microprocessor, which means that a PLC is basically a
specialized computer that has been designed to control the operation of machines and
processes within the harsh environment of the plant.
The language used to program the PLC to perform the logic required to connect the filed
input to its outputs is called Relay Ladder Logic (RLL). The RLL language is programmed by
means of special software using personal computer (connected to the PLC using serial port)
or hand-held programmer which has led or liquid-crystal display and keyboard as illustrated
in Fig. 2.2.
Hand-held programmer or PC communication
Input
Module
(logic or
continuous)
+24V DC
User
interface
CPU & Memory
Output
module
(logic or
continuous
modules)
M
S
N
Serial port
Power Supply
2.3
Fig. 2.2 PLC structure, user communication and interfaces modules
interconnections.
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
RAM and EPROM memory are used to store the program instructions in the PLC. The
computer or hand-held programmer can be used to load and save the RLL programs into the
PLC.
The physical input and output modules can be discrete or analog I/O modules and can be
selected and specified when purchasing the PLC, and depend on the number of the required
I/O lines.
The discrete I/O modules connects field inputs devices of the ON/OFF nature like limit
switches, push button switches, solenoids, solenoid valve or electro-mechanical relay ..etc.
Each discrete I/O module supply voltage source. Since these voltages can be of different
magnitude or types, I/O modules are available at various AC & DC voltages ratings as
shown in Table 2.1. Furthermore, the inputs and outputs are connected to LED’s to indicate
the operation of the I/O module
Table 2.1: common ratings for discrete I/O interface modules.
Interface input module
Interface output modules
24 V AC/DC
12-48 V AC
48 V AC/DC
120 V AC
230 V AC/DC
230 V DC
5 V (TTL)
5 V DC (TTL)
2.1 PLC Operation using scanning technique
A PLC works by continually scanning a program. We can think of this scan cycle as
consisting of 3 important steps, as shown in Fig. 2.3. There are typically more than three
steps but we can focus on the important parts and ignore the others. Typically the others are
checking the system and updating the current internal counter and timer values.
Fig. 2.3 Scanning technique in PLC.
2.4
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Step 1: CHECK INPUT STATUS:
First the PLC takes a look at each input to determine if it is on or off. In other words, is the
sensor connected to the first input on? How about the second input? How about the third. It
records this data into its memory to be used during the next step.
Step 2: EXECUTE PROGRAM:
Next the PLC executes your program one instruction at a time. Maybe your program said
that if the first input was on then it should turn on the first output. Since it already knows
which inputs are on/off from the previous step it, will be able to decide whether the first
output should be turned on based on the state of the first input. It will store the execution
results for use later during the next step.
Step 3: UPDATE OUTPUT STATUS:
Finally the PLC updates the status of the outputs. It updates the outputs based on which
inputs were on during the first step and the results of executing your program during the
second step. Based on the example in step 2, it would now turn on the first output because
the first input was on and your program said to turn on the first output when this condition is
true.
After the third step the PLC goes back to step one and repeats the steps continuously. One
scan time is defined as the time it takes to execute the 3 steps listed above.
The total response time of the PLC is a fact that we have to consider when shopping for a
PLC. The PLC takes a certain amount of time to react to changes. In many applications
speed is not a concern, in others though. The PLC can only see an input turn on/off when it's
looking. In other words, it only looks at its inputs during the check input status part of the
scan.
Fig. 2.4 Variation of scanning time on detecting the high-speed inputs.
In Fig. 2.4 the input 1 is not seen until scan 2. This is because when input 1 turned on, scan 1
had already finished looking at the inputs. Input 2 is not seen until scan 3. This is also
because when the input turned on scan 2 had already finished looking at the inputs.
Input 3 is never seen. This is because when scan 3 was looking at the inputs, signal 3 was not
on yet. It turns off before scan 4 looks at the inputs. Therefore signal 3 is never seen by the
PLC. This illustrates the importance of scanning time of the PLC.
2.5
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
2.2 Understanding Relay Ladder Diagram (RLL)
To understand the programming of PLC relay ladder diagram, let us start with simple case of
relay control system. We can think of a relay as an electromagnetic switch. Apply a voltage
to the coil results in a magnetic field is generated. This magnetic field sucks the contacts of
the relay in, causing them to make a connection. These contacts can be considered to be a
switch. They allow current to flow between 2 points thereby closing the circuit.
Power circuit
Control circuit
Fig. 2.5 Simple control circuit of a bell
Let's consider the following example. Here we simply turn on a bell whenever a switch is
closed, as shown in Fig. 2.5. We have 3 real-world parts; A switch, a relay and a bell.
Whenever the switch closes we apply a current to the bell causing it to sound. The bottom
circuit indicates the DC control circuit. The top circuit indicates the AC control circuit. Here
we are using a DC relay to control an AC circuit. That’s the benefit of using relay. When the
switch is open no current can flow through the coil of the relay. As soon as the switch is
closed, however, current runs through the coil cause a magnetic field to build up. This
magnetic field causes the contacts of the relay to close. Now AC current flows through the
bell and we hear it. Fig. 2.6 shows a typical industrial relay.
Fig. 2.6 A typical industrial relay.
Next, we would like to replace the relay control system with PLC control system using relay
ladder logic. After seeing a few of these it will become obvious why its called a ladder
diagram. We have to create one of these because, unfortunately, a PLC doesn't understand a
2.6
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
schematic diagram. It only recognizes code. Fortunately most PLCs have software, which
convert ladder diagrams into code. This shields us from actually learning the PLC's code.
The PLC doesn't understand terms like switch, relay, bell, etc. It prefers input, output, coil,
contact, etc. It doesn't care what the actual input or output device actually is. It only cares
that its an input or an output.
First we replace the battery with a symbol. This symbol is common to all ladder diagrams.
We draw what are called bus bars. These simply look like two vertical bars. One on each
side of the diagram. Think of the left one as being + voltage and the right one as being
ground. Further think of the current (logic) flow as being from left to right. Next we give the
inputs a symbol. In this basic example we have one real world input. (i.e. the switch) We
Fig. 2.7 Contact relay symbol.
give the input that the switch will be connected to, the symbol shown below. Fig. 2.7 shows
the symbol for contact of switch or relay.
Next we give the outputs a symbol. In this example we use one output (i.e. the bell). We give
the output that the bell will be physically connected to the symbol shown below. Fig. 2.8
shows the symbol used as the output coil or relay.
Fig. 2.8 Output relay symbol.
The AC supply is an external supply so we don't put it in our ladder. The PLC only cares
about which output it turns on and not what's physically connected to it.
Second, we must tell the PLC where everything is located. In other words we have to give all
the devices an address. Where is the switch going to be physically connected to the PLC ?
How about the bell? We start with a blank road map in the PLCs town and give each item an
address. Could you find your friends if you didn't know their address? You know they live in
the same town but which house? The PLC town has a lot of houses (inputs and outputs) but
we have to figure out who lives where (what device is connected where). We'll get further
into the addressing scheme later. The PLC manufacturers each do it a different way! For now
let's say that our input will be called "0000". The output will be called "500", as shown in
Fig. 2.9.
Finally, we have to convert the schematic into a logical sequence of events. This is much
easier than it sounds. The program we're going to write tells the PLC what to do when
certain events take place. In our example we have to tell the PLC what to do when the
operator turns on the switch. Obviously we want the bell to sound but the PLC doesn't know
that.
2.7
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Fig. 2.9 RLL for bell control circuit.
The Fig. 2.9 shows the final converted diagram (RLL) for bell control system. Notice that we
eliminated the real world relay from needing a symbol.
2.3 Basic Instructions of RLL
Main input instructions
Normally open contact :
Normally closed contact :
Main output instructions
Normally open relay
Normally closed relay
Fig. 2.10 Main instructions in RLL.
Example 1:
Draw ladder logic for the control circuit shown in Fig. 2.11:
Fig.2.11 Control and RLL for AND Boolean operation.
The Boolean logic equation for this control circuit: Coil = SW1 . SW2
Example 2:
2.8
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Redraw the relay ladder logic of Example 1, using normally closed switch for SW2 ?
The amended RLL is shown in Fig. 2.12:
Fig. 2.12 Amended RLL using Normally closed switch for switch SW2.
The Boolean logic equation will be : Coil = SW1 . SW2
2.4 Motor control using PLC, two push buttons and motor starter

Here two push buttons
switches (Start/Stop) are used
to switch the motor on/off.
These switches are connected to
the two PLC inputs discrete
type), as shown in Fig. 2.13.
 One of the output ports
(discrete outputs) of the PLC
used to switch motor starter
on/off, which will start/stop the
electric motor.
 The control steps of the
electric motor using RLL is
shown in Fig. 2.14.
Motor
starter
Output
Motor
PLC
Input
Two bush bottom
switches
Fig. 2.13 Motor control using
PLC, motor starter and two
push bottoms.
1
2
3
4
2.9
Go to
step 1
Fig. 2.14 Developed RLL for motor control.
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
2.5 Adding two indicators for the developed RLL
Here two indicators to be added to the developed circuits. Red color to indicates that the
motor is off, and green color to indicates that the motor is running. The amended RLL is
shown in Fig. 2.15.
Motor starter
Output
Motor
Two indicators
PLC
Input
Input
Two push bottom switches
Fig. 2.15 Amended RLL for motor control after adding two indicators.
The modified RLL is given as follows:
Fig. 2.15 (a)RLL when Start push button undressed, (b)RLL when Start push button
pressed
2.10
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
2.6 PLC Programming
The programming technique through serial port via PC and using special software provided
by the manufacturer, as shown in Fig. 2.16 or using hand programmer.
Fig. 2.16 PLC Programming technique using PC computer through serial port.
In case of PC program, a special software program must be installed in the PC which is
provided by the manufacturer to write and edit user Relay Ladder Logic (RLL). This also
transfers the program between PLC and PC computer and vise versa. The program
developed to be friendlier with user during RLL development, as shown in Fig. 2.17.
Fig. 2.17 Example of RLL developed using TPDS software for Toshiba PLC.
Because the PLC uses Relay Ladder Logic diagrams, the convention from any existing relay
ladder to programmed relay ladder logic is simple. Each rung is a combination of input
conditions (symbols) connected from left to right, with the symbol that represents the output
at the far right. The symbols that represent the inputs are connected in series, parallel, or
some combination to obtain the desired logic. The following examples show how PLC can
be used to carry out different control logics, as shown in Fig. 2.18. Note, any combination
logic called Boolean equation.
Example 3:
Develop a relay-ladder logic that allows four switches in a room to control a single light?
The RLL is shown in Fig. 2.19.
SW1
Light
LIGHT=SW1+SW2+SW3+SW4
SW2
SW3
SW4
2.11
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
Boolean or Logic Function
SW1
SW1
SW2
LAMP
{ Relay Ladder Diagram}
LAMP =SW1 and SW2
LAMP= SW1 . SW2
SW2
Lamp
{ Relay Ladder Logic }
LAMP
SW1
{ Relay Ladder Diagram}
SW2
LAMP =SW1 OR SW2
LAMP= SW1 + SW2
SW1
Lamp
{ Relay Ladder Logic }
SW2
SW1
SW3
Lamp
SW1
SW3
Lamp
SW2
SW4
{ Relay Ladder Logic }
LAMP =(SW1 OR SW2) AND SW3
LAMP= (SW1 + SW2) . SW3
SW2
{ Relay Ladder Logic }
LAMP= (SW1.SW3) +(SW2.SW4)+SW5
SW5
SW1
SW3
Y20
SW2
SW4
Y20
Y20=(SW1+ SW2) . SW3
LAMP= SW4 . Y20
Or
LAMP=(SW1+ SW2) . SW3 . SW4
Lamp
Fig. 2.18 Development of RLL for different control logics.
2.12
Manufacturing Automation using PLCs
CHAPTER 2
SW1
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
SW3
Y20
SW2
SW4
Y20
SW1
SW3
SW2
SW4
SW4
SW1
Y20
SW3
Y20=(SW1+ SW2) . SW3
LAMP= SW4 . Y20
Or
LAMP=(SW1+ SW2) . SW3 . SW4
Lamp
Y20
Lamp
Y20
Y20
Y20=(SW1.SW3)+(SW1.SW4)
+(SW2.SW3)+(SW2.SW4)
LAMP= SW4 . Y20
Or
LAMP= ((SW1.SW3)+(SW1.SW4)
+(SW2.SW3)+(SW2.SW4)) . SW4
Y20=SW1. SW3+Y20.SW3
LAMP= SW4 . Y20
Note: 1st Boolean equation is memory or
filp-flop.
SW4
Y20
Lamp
Fig. 2.18 Development of RLL for different control logics
Example 4:
Modify the developed relay-ladder logic given in example 3 such that these switches are
enabled/disabled using external supervisor through switch (SW5) ?
SW1
SW5
LIGHT=(SW1+SW2+SW3+SW4) . SW5
Light
SW2
SW3
SW4
Fig. 2.20 Modified RLL by adding external switch SW5, refer example 4.
2.13
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
PROBLEMS
2.1) Develop the RLL diagrams for the following Boolean equations :
 F1 = A . B + A . B
 F2 = sw1 . sw2 . sw3 . sw4
 F3 = ( sw1 . sw2 . sw3) + sw4
 Y = (A.B.C + D ) . (E.F)
2.2) Drive the Boolean equations for the following relay ladder logics
(a)
SW1
SW3
SW4
Y20
Y20
SW4
Y20
Lamp
(b)
SW1
SW3
SW4
SW6
SW1
Y20
SW2
SW5
2.3) Develop a relay ladder logic that will switch on the motor on/off in automatic and
manual (called jog) modes?
(ans: SW1: START push bottom, SW3 STOP push bottom, SW2 JOG or Manual
operating mode, and Y20 memory that will run the relay that will switch the motor on)
SW1
SW3
Y20
Y20
SW2
Y20
Y20
motor
2.14
Manufacturing Automation using PLCs
CHAPTER 2
Programmable Logic Controller (PLC) and Relay Ladder Logic (RLL)
2.4) Give examples of where a PLC could be used?
2.5) Why would relays be used in place of PLC?
(ans: for some cases is simple and cost effective)
2.6) List the advantages of a PLC over relay control?
2.15