Components in a PLC
PLC's come in different configuration and sizes. Generally PLC's can be separated into
subcategories based on the size of the PLC and the organization of the components in the
PLC. A further division of the PLC unit can be made by dividing the PLC into the working
components of power supply, input and output cards and memory/processor. Input and output
cards are further divided into different categories based on the card type and voltage that the
card uses. When selecting components for a PLC care needs to be taken such that the
components of the PLC meet the requirements of the industrial environment that they are
intended to be implemented in.
Components in a PLC
The programmable logic controller can be broken up into different areas where each area
serves a different purpose for operation of the PLC. Input devices such as contact sensors,
non-contact sensors, analogue inputs and TTL (transistor transistor logic) signals enter the
PLC through a section known as the `input interface'. The input interface is circuitry that is
specially designed to prevent `rouge' signals from damaging the CPU and micro-processor.
PLC microprocessors operate of TTL signals. The TTL signals using in the CPU are either a
5 or 0 volt (Hi or Low) signal. The CPU TTL signal also uses a very low current however the
input field devices are subject to large voltage spikes, noise and current surges, thus the input
interface contains circuitry that prevents signal types that will damage the CPU from reaching
the CPU. The second function of the input interface is to alter the signal from the voltage of
the input device to that of the CPU's TTL signal level.
Once the signal has been modified by the input interface the signal is sent to the processor
unit. The processor unit comprises of several different parts; firstly the power supply takes in
power from alternating current mains or a direct current supply. The power supply then
delivers current to run the processor chips and power input and output interfaces. Some PLC
power supplies can also power small devices requiring a few amps of current (such as small
motors), however this is not common practice. The memory section of the processor
comprises of three different types of chips, the micro-processor area comprises of either
ROM (Read Only Memory) chip types or PROM (Programmable Read Only Memory). This
form of chip is used because ideally the operating system of the PLC should be in a platform
that is stable and does not change. The micro-processor holds the operating system for the
PLC and operates in a fashion analogous to a brain.
The micro-processor directs the operation of the inputs and outputs of the PLC, interprets the
user program and performs computations that are stored in short term memory, RAM
(Random Access Memory). The user program section of the processor is the section where
the ladder logic or user program is stored. Because the ladder logic program is often changed
by the user the chip type used needs to be erasable and reprogrammable. Initially the user
program was stored in the EPROM chip (erasable programmable read only memory), this
chip was erased using ultraviolet light, when the user reprogrammed the ladder logic
program, the EPROM chip was physically removed from the PLC and placed in a ultra-violet
erasing unit to remove the previous program. The chip was then placed back inside the PLC
and the new ladder logic program was then downloaded to the EPROM chip. Because the
process of physically removing the EPROM chip from the PLC was time consuming and
often damaged the EPROM chip, EPROM chip were eventually replaced by EEPROM
(Electrically Erasable Read Only Memory) chips. The EEPROM chip differs from the
EPROM in that, in order, to erase the program in the chip, the chip need not be removed from
the PLC and placed in a ultraviolet erasing unit, but is left in the PLC and a voltage is applied
to designated pins on the chip, erasing the old program. The new program is down loaded to
the EEPROM chip electronically. Reprogramming EEPROM chips with the user's ladder
logic program is now a simple fast process. Both the program in the operating system and the
user's ladder logic program do not change once the PLC program starts executing, however
the execution of the user program generates temporary values for activities as calculations,
states of inputs and output etc. These temporary values are stored in a chip type known as
RAM (Random Access Memory). Random access memory holds values that may be changed
dynamically.
The PLC is programmed using a programming device; there are three main forms of
programming devices

Handheld programmer

Personal computer

Dedicated program loader
The output interface takes signals sent from the CPU and converts them to signals acceptable
for output devices such as motors, solenoids, lights, alarms and TTL activated devices.
Because output devices often contain signals that may damage the CPU the output interface
contains specialized circuitry that protects the CPU from `rogue' signals in the output devices.
The second function of the output interface is to transform the TTL signal from the CPU to
an acceptable signal that will actuate an output device.
Figure: Overview of interaction of hardware components in a PLC
Different categories of PLC based on size and function
PLC's can be categorized on the physical layout of the PLC, the size of the PLC and the
functions which the PLC can perform. Generally there are 5 types of
PLCs

Rack –

Mini PLC

Shoebox

Micro

Software based PLC
PLC operating cycle
The PLC operating cycle is a continuous repetitive process that repeats itself over a period
known as the scan time. The PLC operating cycle can be broken down into 4 separate
processes

Sanity check

Read inputs

Logic Solve

Write outputs
Figure: diagram of sequences in the operating cycle of a PLC.
The operating cycle goes through repetitive cycles. The time for one cycle from one sanity
check to the next sanity check is known as the scan time. This varies from milliseconds to
seconds. The difference in execution of a PLC program and a text program written for a
personal computer in a language such as C, Basic or Pascal is the PLC program automatically
repeats. Text programs written in PLC's do not automatically repeat once the end of the
program has been reached unless they are explicitly instructed to do so. The advantage of the
PLC programming system for industrial automation applications is the inputs and outputs are
constantly scanned by the repetitive nature of the PLC operating cycle. Text based languages
in PLC's are non-repetitive in terms of their operating cycles unless explicitly made to do so.
Input card types
Signals from input devices vary markedly in the signal type, voltage and current of the
incoming signal. A consequence of the diversity of input device signal types is that there are
many different input card types available. There are many trade-offs in selecting input card
types, generally the input card type selected should be dictated by the signal type from the
sensor, however it is possible to select sensor types based on the input cards available for the
PLC. By allowing the input card type to be dictated by the sensor signal, it becomes possible
to select the best sensor for the application and try to obtain the best quality signals for the
PLC. In practice this can be very expensive and most implementations try to minimize the
number of input cards on the PLC by selecting sensors with signals that will minimize PLC
input cards. Input cards generally come in one of the voltages listed below, because these
voltages are often used for industrial switches and sensors.

12Vdc

10-60Vdc

5Vdc (TTL)

100-120Vac

24Vdc

200-240Vac

12-24Vdc

0-10Vdc analogue

4-20mA analogue
Speciality input cards are also available; these input cards are for speciality devices.
Examples are speciality cards for absolute or relative encoders and thermocouples.
Wiring of PLC input cards
Input devices can require a lot of power for operation, as such PLC input cards rarely supply
power, and often external power supplies are required to supply power to inputs and sensors.
The reason for this is that if the PLC unit supplied power for the input sensors then the
current demands of industrial input devices on the PLC power supply is likely to cause failure
of the PLC power supply. As the PLC internal power supply also powers the CPU, any such
fault in the PLC internal power supply would cause the entire PLC to stop functioning. By
having an external power supply, if the power supply fails, then replacement of the power
supply is a simple matter which does not affect the operation of the PLC.
When wiring input cards, the key point to remember is that a circuit needs to be completed.
Completion of circuits require a loop that starts at one terminal of the external power supply
and goes through the sensor/switch to the PLC terminal, the path then goes through the PLC
to the com or V+ terminal and back to the external power supply. Sometimes we have two
different external power supplies wired through the same PLC input card that are used for
different input devices (often with different voltages). If this is the case, circuits must be
completed for each external power supply. Both power supplies can share the some com on
the PLC input card (if the input card is a sinking input card type).
Example 1 – Complete the wiring of the PLC cards, for card a wire up 4 push buttons, for
card B wire up 2 push buttons and 2 contact switches. Note for sourcing input cards the PLC
card acts as a power source. For sinking PLC input cards, the input card acts as a power sink
(ground).
Exercise 2 – Wire up the inputs and outputs on the Zelio-PLC, 4 indicator lights should be
connected to the relay outputs, and 3 push buttons and two contact switches should be
connected to the zelio sinking inputs
Trade-offs in PLC card selection

DC voltages are lower and thus safer

DC inputs are fast

AC is more immune to noise

AC is easier and cheaper

AC signals are common in industrial situations.
Output modules
This section deals with discrete/logical output cards. Two types of PLC output card exist,
discrete/logical output card types or analogue output cards. Discrete output cards send signals
to devices initiating a state in the device as being either on or off, while analogue output cards
send multi-valued signals to output devices.
Output devices differ greatly in the voltage and current required to operate the actuators. A
consequence of this is that output cards for PLCs come in a great variety. Because the current
and voltage requirements of output devices vary greatly it is not practical for the internal PLC
power supply to be used for supply of power for output devices. For this reason, external
power supplies are used to power output devices. The role of the PLC output card is to
provide an interface that switches the external power supply off and on to the output device.
Two methods of selecting output cards exist, the first method involves the selection of the
output actuator and then selecting the output card with the appropriate current and voltage
rating to switch the actuator, however when many different actuators are present, the method
is inefficient on PLC output terminal optimization. The most commonly used method
involves selecting a single PLC output card type and then using the PLC output to switch the
coil on a relay that in turn switches an actuator connected to another power supply.
Presented below are some typical output card voltage types.

120Vac

24Vdc

12-48Vac

12-48Vdc

5V(TTL)

230Vac

0-10Vdc analogue

4-20mA analogue
AC output modules rarely supply power to actuators. External power supplies are used to
power external coils and actuators.
Wiring of output modules
PLC output cards do not supply power to power output devices. Output devices such as
motors can require many amps for successful operation. Furthermore changes in the
operating loads and starting load requirement of a motor can introduce voltage spikes and
current surge conditions. These load conditions are unsuitable for a PLC's internal power
supply and as a consequence PLC power supplies do not supply power to output devices
through the PLC output cards.
Output devices must be supplied with power using external power supplies. The role of the
PLC output card is solely as a switching element that switches power from the external power
supply (that drives the output device), on or off. Like input modules, output modules require
completed circuits. Power from the power supply must pass from the power supply terminal
to the actuator, into the PLC card terminal, back to a com or V+ terminal on the PLC output
module, and then back to the other terminal of the power supply. The output module for a
PLC can also be a relay output, this can switch both AC and DC, however it is slower
switching (10ms), other output modules types use triacs for switching AC. PLC AC output
cards that contain triacs use a single shared terminal that is connected to the power supply.
The single shared terminal is wired directly to the external power supply. The remaining
terminals on the PLC output card are then connected to devices, and the device is connected
to the other terminal of the external power supply. PLC Relay outputs by contrast have two
unique terminals for switching each output device. A consequence of this is relay output
cards can switch half the number of output devices as triac output cards. i.e. a triac output
card with 9 connections can switch 8 devices, while a relay output card with 8 connections
can switch 4 devices.
Example: wire up the sinking and sourcing output cards, connect 3 solenoids and 3 indicator
lights to card a). Connect 4 solenoids and 4 indicator lights to card b).
Why card A is sourcing?
Why card B is sinking?
Sinking sourcing concepts will be covered in more detail in future classes.
Output module components
There are broadly two classes of output cards

Dry contacts

Switched outputs
Dry contacts use a relay for each output device. Each relay output is capable of switching
both AC and DC external power supplies. Switched outputs by contracts use solid state
devices. Output cards containing transistors can only switch DC external power supplies and
output cards containing triacs can switch only AC external power supplies. The advantage of
using solid state switching devices is the response time is considerable faster than (>1ms)
than dry contact switching.
Example: Wiring diagram using a relay output to switch
1) a DC solenoid
2)a AC heater
3) an external DC contactor that supplies a large amount of current to a motor from an AC
power supply (picture adapted from a picture in Hugh Jack, Systems Integration)
Relays
External relay devices are often used when we have a PLC that can switch a power supply
operating off one voltage and we have an actuator that uses another voltage. In this case the
output circuit from the PLC card runs through the coil on the external relay. The contactor
side of the external relay is then used to switch a device from another external power supply
(operating at a different voltage to the PLC output card). Using this method a device can be
switched by the PLC output card where the device is operating off a different voltage. This
method can even be used where the PLC output card is AC and the actuator is DC or vice
versa. A second reason for using the PLC output to switch an external relay that then
switches the output device is to protect the PLC output card circuitry from the current and
voltage demands of the actuator. Some actuators require large starting currents or are prone to
producing voltage spikes in circuits. By using the external relay as the switching element for
the actuator the PLC output card circuity is isolated from the current and voltage demands of
the actuator. All
PLC output cards and output circuitry has a maximum rated current, by using the external
relay, an output can be switched that exceeds the PLC's rated output card current.
Relay terminology
Relays are generally used for switching large loads and for control logic. When selecting
relays there are a number of terms that are used, some definitions for these terms are
presented below
 Contactor
 Arc suppression

AC coils
 Rated voltage
 Rated current
When selecting relays, it is important to pay consideration to the rated values.
The three most important rated values are the voltage type. If the relay coil is rated for DC
then AC should not be used and if rated for AC then DC should not be used to power the
relay coil. The second rated parameter is the rated voltage. The relay will usually specify two
rated voltage values, one for the coil and one for the contactor. If the rated voltage is
exceeded then either the coil or contacted will fail depending on where the rated voltage is
exceeded. If the rated voltage is exceeded on the contactor side, arcing occurs and carbon
build-up starts to appear on the contacts eventually stopping the contactors from switching
the circuit. If the voltage applied to the coil is considerable less than the rated voltage then the
coil will have insufficient voltage to switch the contactors and will either switch
intermittently or not switch at all. The rated current value apply to the contactors, if the rated
current is exceeded the contactors will over heat and the relay will stop functioning. In the
worse case scenario the contactors may weld shut and will not be able to change state with a
change in voltage across the coil.
Example: Connect a DC indicator light to Q1, an DC solenoid to Q2, Q3 takes power from a
24VDC power supply and activates a relay that then switches a 230VAC motor. I1-I3 are
pushbuttons.