PLC input and output (I/O) devices

Input/output (I/O) is information representing the data that is received from senses elements / devices
and the commands that are sent to actuating and indicating devices.

The term sensor is used for an input device that provides a usable output in response to a specified
physical input. For example, a thermocouple is a sensor which converts a temperature difference into an
electrical output.

The term transducer is generally used for a device that converts a signal from one form to a different
physical form. Thus sensors are often transducers, but also other devices can be transducers, e.g. a
motor which converts an electrical input into rotation.

The number of I/O devices used within a control system is called its point count. Thus the total number
of digital and analog point is used to give an indication of the size of a control system.

I/O modules connect "real world" field devices to the controller. They convert the electrical signals used
in the field devices into electronic signals that can be used by the control system, and translate real
world values to IO table values.

I/O modules communicate with PLC CPU in one of three ways:

1. Backplane - The I/O modules can be located in the same rack or station. Communications then
takes place within the rack or across the backplane.

2. Backplane extension - backplane extension modules allow I/O modules to be located in racks
or stations which are separated from the controller.

3. Device network - modules can communicate with a controller over a network. Industrial
networks are used to interconnect field level devices with controllers. Common IO networks are
FieldBus, Profibus, and DeviceNet.

There are major types of I/O

• Analog – continuous devices that sense and respond to a range of values.

• Digital – binary devices which must be in one of only two states on or off.

1. Analog input and output devices

- Analog input devices senses continuous parameters common analog inputs are pressure, temperature,
speed transducers etc.

- An analog input card converts a voltage by current leg or signal that can be anywhere from 0 to 20mA
into digitally equivalent number that can be understood by the CPU.

- To input an analog voltage (into a PLC or any other computer) the continuous voltage value must be
sampled and then converted to a numerical value by an A/D converter.

- Analog output devices respond to a range of output values from the controller common analog output
signals include motor speed, valve position, air pressure etc. An analog output card will convert a digital
number sent by the CPU to its real world voltage or current. Analog device data requires significantly
more manipulation and processing then digital device data.

2. Digital input and output devices

- Inputs come from sensors that translate physical phenomena into digital signal. Thus digital input
devices may be either on or off, they may not hold any other value.

- Common digital field input devices include push buttons, unit switches and photo eyes. - Digital output
devices are devices which give either on or off. Common types are relays, motor starter, solenoid valves
etc.

Examples of inputs and outputs

- Inputs for a PLC come in a few basic varieties the simplest are AC and DC inputs. Examples of input
devices are:

• Proximity switches – use inductance, capacitance or light to detect an object logically

• Switches – mechanical mechanisms will open or close electrical contacts for a logical signal

• Potentiometer – measures angular position continuously using resistance.

• LVDT (Linear variable differential transformer) – measures linear displacement continuously

- Outputs to actuators allow a PLC to cause something to happen in a process. Outputs from PLC are
often relays, but they can also be solid state electronics such as transistors for DC output or TRIACs for
AC outputs. Continuous output requires special output cards with digital to analog converters.

- Examples are

• Solenoid valves – logical output that can switch a hydraulic or pneumatic flow

• Lights – logical output that can often be powered directly from PLC output boards

• Motor starters – motors often draw a large amount of current when started, so they require
motor starters which are basically large relays.

• Servo motors – a continuous output from the PLC can command a variable speed or position.

Exercise describe using diagrams the way in which d.c devices are connected to a PLC using sourcing and
sinking configurations?

Types of PLC system

• The PLC sizes are given in terms of program memory size and the maximum number of I/O points the
system can support.

• However to evaluate properly any PLC, consideration is taken for many additional features such as its
processor, cycle time, language facilities, functions expansion capability etc.
1. Small PLC – small and mini PLCs are designed as robust, compact units which can be mounted on or
beside the equipment to be controlled. They are mainly used to replace hard wired logic relays, timers,
counters etc that control individual items of plant or machinery, but can also be used to co-ordinate
several machines working in conjunction with each other. Programming is by way of logic instruction list
(mnemonic) or relay ladder diagrams.

2. Medium-sized PLC: - In this range, modular construction predominates with plug-in modules on rack
mounting system or Back plane system. This allows the simple upgrading or expansion of the system by
fitting additional 1/0 cards into the racks

3. Large PLC - where control is very large numbers of input and output points is necessary or complex
control functions are required, a large PLC is selected. It is designed for use in large plants or machines
requiring continuous control. They are also employed as supervisory controllers to monitor and control
several other PLCs or intelligent machines e.g. CNC tools.

PLC styles of construction

• The main styles are unitary, modular and rack mounting.

1. Unitary PLC - is the smallest and least expensive. It contains every feature of a basic system in one box
and is attached to the machine being controlled. They are not expandable so the application is limited to
on-board I/O.

2. Modular – These are a range of modules that slot together to build up a system. Basic modules are
the power supply, the main module containing the CPU, the input module and the output module.
Modular PLCs are used in applications where a higher I/O count is needed or when using specialty
modules such as quadrature encoders. The main advantage is that the number of input and output
terminals can be expanded to cope with changes to the hardware system.

3. Rack mounting – are usually more expensive, expandable and powerful than modular PLC. The rack
provides a power and communication backplane that greatly increases the communication rate between
the processor and the modules as well as allowing some specialty modules to communicate with each
other without the processor. The number of available 1/0 points is also much higher in the rack systems.
PLC Programming

- Programming devices can be hand-held devices, a desktop console or a computer. Only when the
program has been designed on the programming device is ready, it is transferred to the memory unit of
the PLC

• Hand-held programming devices – will normally contain enough memory to allow the unit to
retain programs while being carried from one place to another.

• Desktop consoles – are likely to have a visual display unit with a full keyboard and screen
displays.

• Personal computers – are widely configured as program development work station. A major
advantage of using a computer is that the program can be stored on the hand disk or CD and
copies easily made.

The following is a list of programming languages.

i) Ladder diagram (LD)

Ladder diagram Language

- A ladder diagram is a symbolic representation of an electrical circuit. Thus the symbols utilized closely
resemble schematic symbols for electrical devices.

- This language is a symbolic instruction set that is used to create PLC programs. The ladder instruction
symbols can be formatted to obtain the desired control logic, which is then entered into memory.

- The industry trend is toward using the IEC 61131-3 standard, though a voluntary standard; individual
manufactures here some freedom in the implementation.

- Other PLC manufacturers are Allen-Bradley, Control Logix, Modicon, Siemens 57 etc.

Ladder logic symbols

1. The basic ladder logic input symbols are

2. The basic ladder logic coil (output) symbols

- By using this approach, the engineers and technicians using the early PLC did not need retraining to
understand the program.

- The use combinational logic where the output is purely dependent of the combination of inputs at any
instant in time. They use AND, OR, NOT, X-OR to create ladder logic. In all the ladder logic, symbols are
used for all inputs, outputs and internal memory.

- Vertical lines on the left and right are called the power rails. The contacts are arranged horizontally
between the power rails, hence the term rung.

- The main functions of a ladder diagram program are to control outputs and perform functional
operations based on input conditions. Ladder diagrams use rungs to accomplish this control.

- In general, a rung consists of a set of input conditions (represented by contact instructions) and an
output instruction at the end of the rung (represented by a coil symbol). The contact instructions for a
rung may be referred to as input conditions, rung conditions, or the control logic.

Ladder diagram rules

- A ladder diagram is read from left to right and from top to bottom.

- The vertical power lines or rails may be labeled L1, L2 or X1, X2 when the voltage potential is derived
from a transformer.

- Devices are shown in order of importance whenever possible. Stop button should be given a higher
order of importance.

- All contacts associated with a device change state when the device is energized.

- Devices that perform a stop function are normally placed in series on a rung.

- Devices that perform a start function are normally placed in parallel or in a branch configuration.

- Contact associated with relays timers, motor starters always have the same number or letter
designation as the device that controls them.

Example 1

Two switches labeled A and B are wired in SERIES controlling a lamp. Implement this function as PLC
ladder logic where the two switches are separate inputs.
Assignment

Questions

1. Devise a ladder diagram for a system where there has to be no output when any one of four sensors
gives an output, otherwise there is to be an output.

2. A signal lamp is required to be switched on if a pump is running and the pressure is satisfactory, or if
the lamp test switch is closed. Draw a ladder diagram.

3. Consider a valve which is to be operated to lift a load when a pump is running and either the lift switch
is operated or a switch operated indicating that the load has not already been lifted and is at the bottom
of its lift channel. Devise a ladder diagram.

4. An Alarm system is used in conjunction with an automated bottling system in a milk bottling plant. A
conveyer belt carries empty bottles that are to be filled with milk. The alarm goes off in any of the
conditions occurs.

- Milk tank is empty and bottles are in conveyor belt.

- There are no bottles in the conveyer and there is milk in the tank.

- There is milk in the tank and bottles on the conveyor belt but electric power is off.

- There is no milk in the tank, no bottles on the conveyor belt and electric power is off.

i) Write down a Boolean expression for the alarm system.

ii) Implement this system using a PLC ladder diagram.

5. Draw the ladder rungs to represent:

a. Two switches are normally open and both have to be closed for a motor to operate.

b. Either of two, normally open, switches have to be closed for a coil to be energized and operate
an actuator.

c. A motor is switched on by pressing a spring-return push button start switch, and the motor
remains on until another springreturn push button stop switch is pressed.

d. A lamp is to be switched on if there is an input from sensor A or sensor B

e. A light is to come on if there is no input to a sensor.

f. A solenoid valve is to be activated if sensor A gives an input.

SET and RESET

- Another function which is often available is the ability to set and reset an internal relay. The SET
instruction causes the relay to self-hold, i.e. latch. It then remains in that condition until the RESET
instruction is received. The term flip-flop is often used.

- The SET coil is switched on when power is supplied to it and remains set until it is RESET. The RESET coil
is reset to the off state when power is supplied to it and remains off until it is SET.
- SET and RESET instruction will change the status of bit operations only when the implementation is ON.
In the OFF condition, the instructions will not change the bit operation status.

Timers

- In many control tasks there is a need to control time. Timers are devices that count increments of time.
PLCs thus have timers as builtin devices. Timers count fractions of seconds or seconds using the internal
CPU clock.

- The way the timers work varies from one type of PLC to another. A common approach is to consider
timers to behave like relays with coils which when energized result in the closure or opening of contacts
after some preset time. The timer is thus treated as an output for a rung with control being exercised
over pairs of contacts elsewhere.

- The timer compares its current time with the preset time. The output of the timer is a logic 0 as long as
the current time is less than the preset time. When the current time is greater than the preset time the
timer output is a logic 1.

Assignment

Two motors (M1 and M2) are to be controlled as follows:

- When the run switch is operated both motor must run

- After 4 min motor 1 must stop

- Motor 2 continues running for another 2 min and stops

- At this point a lamp is switched on - After a further 90 sec, the lamp goes off and the cycle
restarts

- If a stop switch is operated at any time, the system will continue to the end of the cycle and
then stop.

Produce the PLC program

Counters

- Counters used in PLCs serve the same function as mechanical counters. Counters compare an
accumulated value to a preset value to control circuit functions.
- Counter is used to count and store the number of occurrence of an input signal. Control applications
that commonly use counters include the following:

• Count to a preset value and cause an event to occur

• Cause an event to occur until the count reaches a preset value

- Counters increment/decrement one count each time the input transitions from off (logic 0) to on (logic
1).

Questions

1. Components pass along a chute and interrupt a light switch which goes low (off) each time it is
interrupted. Every time 6 components have been counted, an eject operation is used to remove the
batch and the then it all starts again. Produce a ladder logic diagram to do this operation. The counter is
designated C460

2. Design a ladder program for an industrial control system that:

• Count ten objects passing along a conveyor belt;

• Closes a deflecting gate when the number has been deflected into a carton

• Allows a time of 5 seconds between the tenth object counted and closing of the deflector.

3. A controlled car park has 4 spaces in the packing lot. Cars are detected and allowed to enter into the
parking space if available. If NO space a “Full” indicator lamp should be lit, otherwise individual indicator
lamps should light to show the available parking space. Design a PLC ladder diagram of the car parking
system; include comments on every rung.

4. With the aid of a ladder program and a process control figure, explain how a converter can be used in
a machine to direct 6 products to a packaging box and 12 products to another box simultaneously.