UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
1. Describe the design characteristics of a modular PLC, and compare it to unitary
(brick) and software based PLCs.
A modular PLC contains multiple modules or “cards” that can be compiled together to build a
bespoke purpose made controller. Generally, a base module/card such as electrical power
regulation, the computers processor and input connections. Other cards such as analog to
digital converters. Also, more outputs can be added when required. This design means that the
PLC can be modified and built easily.
A common modular PLC unit is capable to handle between 23 and 40 inputs and outputs as
standard. The number of connections can be increased by adding more cards, this gives lots of
flexibility and range for software programs.
A unitary (brick) PLC is a more simplistic type of controller, it contains all necessary system
components like the processor in just one box that runs the software program (as well as ports
for the input and output connections). Unitary PLCs are usually connected directly to the
equipment it is controlling.
A common unitary PLC unit includes built in memory for storing programs, 32 digital input/output
ports and a communication port used in programming the unit.

Memory
Unitary PLCs are only capable of holding an (x) amount of information. This is because all
necessary parts are kept in the single housing. Additionally, there isn’t much room for expanding
information past the basic functions it requires to work. Modular PLCs have lots more memory
and has the capability to keep higher volumes of information stored.

I/O Modules
The unitary PLC has limited input and output modules resulting in limited performance. Also, if
the need to expand your operations at work the unitary PLC will be incapable of achieving this.
Modular PLCs have the capacity to do more complex processes as you can just keep adding
I/Os. The modular PLC was designed for the purpose of growth as you can expand and
customize them.
UNIT 22: Programmable Logic Controllers – Assignment 1

Phil Bishop
Troubleshooting
Even though Unitary PLCs do not alert the operator if there is a fault, leading to longer
downtime and potential difficulties in fault finding. They are considerably cheaper to buy.
Modular PLCs can easily troubleshoot problems and even keep some processes functioning
when fixing the fault(s). in terms of longevity modular PLCs can help save money by reducing
downtime and specifying faults.

Interchangeability
The cards used in Modular PLCs can be swapped and changed due to pluggable I/O cards.
This gives great customizing capabilities, a diverse experience and the potential to create more
complex processes than Unitary PLCs can.
2. Describe the internal architecture of a typical modular PLC. Include a description
of the role of each component part and a diagram of how they interact with each
other.
Modular PLCS are adaptive systems that can be modified and interchanged easily. They consist
of a chassis rail where a collective of modules/cards are built into an interfaced system. The
components needed for a functioning system are, CPU, power supply and I/O modules. One of
the main advantages of a Modular PLC is its architecture. It gives the user the ability to choose
individual specifications as required. Such as, the quantity and type of the input/output cards
and also you can include network interface cards.
Modular PLCs are connected by a motherboard that has a comms link between all the cards,
letting the CPU register where and what is connected. I/O modules can take out the need for
wiring by networking process signals to digital communication links. Retrieving the data is done
by the standard sensors and transmitters mounted in the module.
Internal PLC architecture like processing power, memory consumption, I/O devices and current
load will impact the system. The CPU inside the PLC is a microprocessor. The type of processor
needed will depends on what design is implemented, like complex logic operations, local area
network accessibility and user interface. Data is sent across the data bus in the back plane.
Generally the process follows, scan starting, internal checks performed, scan inputs, execute
program logic and then update outputs.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
3. Describe the CPU operation “mass I/O copying”
At the start of each program cycle the CPU scans all the inputs and copies their status’ into the
input and output addresses in random access memory. As the program is carried out the stored
inputs data is read from the RAM, then the logic operations are performed. The resulting output
signals are stored in a reserved area of the RAM. At the end of each cycle the outputs are
transferred from the RAM to the channels and keep their status until the next update.




Scan all inputs and copy into ram
Fetch, decode and perform all program functions in sequence (copying output
commands into RAM)
Update all outputs
Repeat Cycle
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
4. The system can also be controlled by an embedded controller
a. Briefly describe what a PIC controller is.
b. Determine whether the current modular PLC or a PIC is the best option for their
system.
A PIC (Programmable Interface Controller) is similar to a PLC in which electronic circuit can be
programmed to carry out a range of tasks. They are commonly found in devices such as
computers, smart phones and alarms. They contain a processor and memory bank. Computer
software is required to program PIC’s.
Modular PLCs are industrial grade computers. PLCs are commonly powered by 240v domestic
supply. PLCs accept inputs and give outputs which can be 24VDC for sensors and solenoids.
Modular PLCs communicate to with each installed card and can even offer Ethernet capabilities.
PLC inputs and outputs are usually protected reducing the possibility of damage or interference.
PLCs are also very interchangeable and can be bought off the shelf and programmed.
A PIC is a very small controller. This would require an adequate circuit build to suit the PIC to
work similarly to that of a PLC. PICs also run off of low DC voltage and only capable of handling
up to 20mA meaning you will need to step down supply voltage even further, this also means
you cannot connect sensors or solenoids directly to the PIC. Due to the size of PICs they will be
costly to have manufactured.
For the purpose of running this system I would choose the current Modular PLC as it is more
versatile to use and requires less preparations for the system to function effectively. The signals
from the flow meter, temperature probes and drain solenoid can be sent and received directly to
the PLC whereas it can’t with the PIC.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
5. Describe how the following devices measure or operate, and in each case state
the correct IO card that will connect them to the PLC
a. Temperature probe
b. Flow meter
c. Drain solenoid
Temperature probes work by being placed in the proximity where the temperature needs to be
recorded. The temperature is read by the change in resistance due to the resistance being
affected by temperature (as the temperature changes the resistance increases/decreases). A
formula is used to interpret the change in resistance to correlate with temperature. An Analogue
IO card will be required to let the PLC convert the signal into a value.
Flow meters are devices that measure the amount of liquid (or gas) that passes through them.
The measurements can be calculated in periods of time or total volume (100l per minute or
high/low flow). Magnets are used to cause a voltage signal that is picked up by the sensor,
when the flow increases the voltage signal gets stronger, the signal is then converted into a
figure representing flow rate. An Analogue IO card will also be needed for the flow meter as it
works on a similar principle to the temperature probe
Drain solenoids work by having an electromagnet as a housing for a magnetic plunger. Once a
voltage is applied to the solenoid it activates the electromagnet pulling up the plunger, this
plunger is mechanically connected to the drain valve forcing it to stay open until the voltage is
removed. Only a Digital IO card will be required for the drain solenoid as all it needs is power to
open the valve, it doesn’t need to convert any signals.
6. The flow meter will provide analogue signals to the PLC, but the diverter solenoid
will only require a digital one. Explain why.
The solenoid only requires a digital output because its function is very basic. Digital outputs only
work one of two days, ON or OFF. If the valve needs to be open the output will be ON if the
valve needs to be closed the output will be OFF.
Analogue signals are variable, they can have different states. Analogue inputs are used for
things like temperature or flow rate because the voltage received back is converted using
formulae to represent a figure. Also analogue outputs can be used to actuate devices partially.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
7. The temperature probe will provide an analogue input to the PLC, assume the
parameters have been set to measure -5 degrees and +105 degrees.
a. Use a variable signal of your choice over 100ms, show the digital conversion
using 2 bit resolution and 5ms sample rate.
b. Using the same variable signal, show the digital conversion using 4 bit resolution
and 5ms sample rate
c. Comment on the differences between the two conversions
mA
4-9
9 - 15
15 - 18
18 - 20
Temperature °C
-5
31.6
68.2
105
2 Bit = 22
-5 to +105 = 110
= increments of 36.6
mA
4-5
5-6
6-7
7-8
8-9
9 - 10
10 - 11
11 - 12
12 - 13
13 - 14
14 - 15
15 - 16
16 - 17
17 - 18
18 - 19
19 - 20
Binary
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Temperature °C
-5
2.3
9.6
16.9
24.2
31.5
38.8
46.1
53.4
60.7
68
75.3
82.6
89.9
97.2
104.5
4 Bit = 42
-5 to +105 = 110
110 / 15 = increments of 7.3
The differences between these two conversions (shown on attached excel file) show that having
a higher number of bits will provide more accurate readings of temperature. The change in
potential difference in the sensor is affected by temperature, the more bits that are available
mean that more changes in the potential difference can be measured, resulting in more
accurate temperature readings.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
8. The flow meter has provided a range of values that have been stored by the PLC
in a variety of different formats show below. Complete the following table (show
your working for two different rows with different starting numbering systems).
Decimal
Binary
Octal
Hexadecimal
7
216
33
167
52
188
424
5
74
311
0111
11011000
0100001
10100111
00110100
10111100
0000000110101000
101
01001010
100110111
7
330
041
247
64
274
650
5
112
467
7
D8
21
A7
34
0bc
1a8
5
4a
137
Binary Coded
Decimal
0111
0010 0001 0110
0011 0011
0001 0110 0111
0101 0010
0001 1000 1000
0100 0010 0100
0000 0000 0101
0111 0100
0011 0001 0001
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
9. Coaxial cable, twisted pair and optical fibre can all be used to form
communication links. Describe how each one works and determine the best one
to use to connect the cooling system to the main office.
Coaxial cable is comprised of a single core conductor sheathed by a dielectric insulator, which
is protected by a woven copper shield enclosed by an outer plastic shield. The cable is
specifically designed like this with precise constant conductor spacing which is needed for it to
function effectively. The electrical signal is conducted using the inner single core, the
electromagnetic fields are restricted to the woven copper shield which pacifies interference.
Twisted pair cable is a form of wiring where two conductors of the same circuit (but are carrying
equal and opposite signals) are twisted together. Similarly to Coaxial cable, this is done to
pacify the effects of electromagnetic interference.
Optical Fibre cable contains several cores called “optical fibres” these cores are used to carry
light. These cores are coated in plastic sheaths and enclosed in a tube suited to the
environment it’s required to be in. the cable is manufactured for the purpose. For example, you
can have one design of cable for high speed data transfer and a different design for
telecommunications.
I would choose a Coaxial cable for the purpose of connecting the cooling system to the main
office because the design of the cable is a much more practical design for travelling through the
environment it will be placed in.
60 meters is a reasonable distance for the cables to travel, excluding the twisted pair cables.
This type of cable usually has a very small cross sectional area and is only used for local
communications, as the cable needs to be travelling to the central PLC (although it is good as
avoiding the interference induced by the electrically noise area). I feel that it is too small to travel
60 meters due to potential signal drops and vulnerability to physical damage.
With the Optical Fibre cable, 20 meters of an electrically noisy area is more than capable of
disturbing the efficiency and effectiveness of the cable. Also the environment may mean that the
Optical Fibres are susceptible to dynamic fatigue as well as static fatigue. Although it offers the
fastest data transfer the environment has to be correct for the cables capabilities to thrive.
The Coaxial cable offers a large cross sectional area due to its layers of protection which make
it ideal for travelling 60 meters to the main office. Additionally, these layers of protection will not
only offer physical protection for the communications but also static protection, the woven
sheath and plastic shield suppress electromagnetic interferences created by an electrically
noisy environment.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
10. Assuming the communication cable is coaxial; describe which one of the
following protocols would be suitable and why the others are not.
a. RS232
b. IEE488
c. 20mA Current Loop
I think that the 20mA current loop protocol is not suitable for this purpose because, although it
has advantages such as immunity to electrical noise, this is only obtainable through being in a
twisted pair circuit. Also for 20mA current loop connections to work an end to end DC continuity
needs to be established, these connections cannot be made with cables such as multiplexed or
fibre optics. This also means that the supply is not isolated from the output. Even though the
20mA current loop keeps the current stable, it is prone to voltage drops over long distances.
Additionally the 20mA current loop protocol has high power consumption compared to other
analogue signals.
I feel that the IEE488 protocol is unsuitable for this purpose because of its intended purpose.
This protocol is only used for short ranged connections, rendering it unsuitable for travelling the
distance to the main office. Although you can have multiple instruments on a single bus, if you
do not expand it this amount is limited due to the length of the connection (20m or 2x(number of
devices) whichever is less). Additionally this protocol generally has low bandwidth meaning slow
connection or response time and bulky connectors often resulting in poor cable reliability, which
is essential for the main PLC.
I think that the RS232 protocol is the most suitable for this purpose because of its popularity,
most controllers will have this connection making connectivity easier. Additionally the use of
Coaxial Cable will work with an RS232 protocol, even though it is intrinsically unbalanced the
coax cable insulation will stop this. As this protocol is asynchronous there is no separate
synchronizing signal, which means it will synchronize itself. Once this is achieved serial
communications transmit one bit at a time (where as parallel transmission transmits all at once)
because the serial transmission happens one bit at a time only a single cable is required, which
is the configuration of coaxial cable. This type of data transfer is also preferable at long range,
which makes it the most ideal protocol for connecting to the main office.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
11. Determine which of the following methods “master-slave” or “peer to peer” would
be suitable.
In networking master-slave is a type of communication protocol ford hardware devices in which
one device has full control over one or more other devices. All controls are routed from the
designated “master” and the “slaves” carry out the processes.
Peer to peer is a decentralized communication protocol in which each device has the same
capabilities as the other and either one can initiate communications. In this protocol the client
makes a service request and the server carries this out. In this configuration the device can
function as either the client or the server.



Peer to peer method may be more resilient to failure than the master-slave method. If
the master devices is somehow unavailable then the slaves do not receive command
and stop their processes (a standby master would need to be put in place).
If the master device receives a fault sent back by one of the slaves it should command
all other slaves to stop their processes to prevent potential accident from happening.
Both methods are credible and have attributes which may appeal more to certain
requirements. If high availability is wanted then the peer to peer method may be best.
But if all data is oversaw by a central device then the simpler master-slave method could
be best.
Due the central PLC in the main office is used to oversee the processes carried out by the other
PLCs I feel that this is the exact purpose of the master-slave method. Each PLC will receive its
command based on what is fed back to the master PLC, making the entire process work with
each other smoothly via the master PLC.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
12. Describe why the networking standards ISO/OSI 7 Layer Model is useful to link all
of the PLCs in the system.
In the OSI 7 layer model, control is passed from one layer to the next, starting with the
application layer, working its day to the bottom and back up again. The OSI 7 layer model takes
on the role of inter-connecting networks and separates them into a “vertical stack” which
consists of the 7 layers.

-
Physical Layer
This layer passes the signal through the network at the electrical and mechanical level. It
provides the hardware the means of sending and receiving the data via physical cables
or cards, Ethernet or RS232.

-
Data Link Layer
In this layer the data packets are encoded and decoded into bits. It furnishes
transmission protocol knowledge as well as managing and handles errors in the physical
layer, flow control and frame synchronization.

-
Network Layer
This layer provides switching and routing technologies, creating logic paths for
transmitting data from PLC to PLC as well as routing and traffic controlling paths.

-
Transport Layer
This layer provides transparent data transfer and is responsible for error recovery and
flow control ensuring the complete transfer of data.

-
Session Layer
This layer creates, maintains and ends connection between applications. The session
layer sets up, “conversations” “exchanges” and “dialogues” between applications.

-
Presentation Layer
This layer provides independence from differences in how data has been represented by
translating from application format to network format (and vice versa). This layer
transforms data into a form that can be accepted (as well as encrypting data that is sent
across a network, providing freedom from compatibility issues).

-
Application Layer
This layer supports end-user processes. User identity and privacy are considered,
communication partners and quality of service is identified as well as presentation layer
issues (received encrypted data). Everything on this layer is application specific.
UNIT 22: Programmable Logic Controllers – Assignment 1
Phil Bishop
With each layer process identified and summarized it is clear to see why the ISO/OSI 7 Layer
Method is useful to link all the PLCs in the system together. Each layer conducts a process
which is essential for all the PLCs to function correctly. With receiving a signal on a
mechanical/electrical level it means that a sensor has been triggered (regardless of whether to
start or stop the process). This data is then sent through the connection and is identified
resulting in control of its destination, as well as switching and routing to go to the correct PLC
further supported by the Transport and Session layer.
This is proving that the layers in this module are carrying out exactly what is needed to be
established across all the PLCs as well as the Central PLC. Where data encryption, routing,
security, signal receiving and commands are communicated appropriately across the whole
network, keeping a healthy link across all the connections.