PLC Assignment: Types, Input and Output Devices, Communication Links, Programming Languages, and Internal Architecture

This assignment discusses the types of PLCs, input and output devices used with PLCs, communication links, programming languages, and internal architecture of PLCs.

PLC ASSIGNMENT
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Task 1
Compact/block type PLCs are designed to perform
elementary functions. Since they are small, the power supply,
I/O system, and the power supply units are housed in one
structure. On the other hand, modular or rack type PLCs have
numerous bases allowing installation of multiple and
independent modules.
Compact PLCs are cheap in cost since they offer less
in their functionality. However, modular type PLCs offer
much more making them expensive. As a result, rack type
PLCs they have many advantages over block PLCs. First, they
have larger memory there is more room to accommodate
larger volumes of information. Second, unlike compact PLCs,
they have many and expandable I/O modules to undertake
many complex processes. Besides, they have a larger room for
expansion with more ease in troubleshooting and less
downtime. This implies that some processes can be in
operation while fixing the faulty part. While compact PLCs
are characterized by lower initial capital outlay, modular
PLC’s have a longer-economic security making them more
efficient [3].
Task 2
Input devices used with PLCs include sensors,
switches, encoders, scanners, keypad, computers, and
touchpad Besides, the output devices include contactors,
relays, drives, valves, alphanumeric display, and motor starters
[4].
Input devices such as sensors detect a physical
quantity and converts into an electrical quantity such as
current. For example, thermocouples which convert the
temperature difference into an electrical quantity. These
electrical values are fed to the PLCs’ input ports. Output
devices, on the other hand, receive commands from the PLC
depending on the level of control required and the desired
output. They turn a device on or off depending on the control
instruction. For instance, increasing or reducing the speed of a
motor.
Task 3
PLCs offer a variety of communication links to be
used when they need to connect to other external devices.
First, host computer links can connect intelligent devices to
PLCs. In fact, most devices are made to connect to host
computer networks through ports such as RS422. All PLCs in
the system are connected to the host computer in a daisy-
chain. As a result, the PLC can acquire data and send a signal
to the remote devices. Second, Peer-to-peer networks enhance
communication by the duplication of common memory. In this
way, data written to a PLC’s memory is automatically copied
to other PLCs within the network. Besides, remote I/O
systems enhance communication when the input and output
devices are separated by longer distances. In such
configurations, the I/O devices are connected to the PLC
through fiber optic or twisted pair cables. Notably, the master
controller checks the current status of I/O devices any signals
them to change output states as instructed by the PLC [2].
Task 4
The IEC61131 specifies five programming languages
for use in PLCs [5]. First, two graphical programming
languages which include Functional Block Diagram (FBD)
and Ladder Logic (LD). In fact, LD is the common PLC
programming language. It is based on relay logic in electrical
circuits making it easy to follow. It resembles control circuits
where a series of outputs are required to activate an output.
FBD is the second most widely used PLC programming
language. It uses instructions which are similar to Ladder
making much easier to understand. FBD is commonly used in
simpler programs that involve digital inputs such as the
Infrared sensor. Its closeness to Ladder makes it applicable
wherever Ladder works.
Second, there is the Sequential Function Chart (SFC)
that closely resembles the familiar flowcharts. The flowcharts
are followed by transition stages showing all steps in the
program. Its visual nature makes troubleshooting a problem
friendly and easy for the maintenance crew. For this reason, it
is common to find it applicable to sequential operations.
Then, two textual languages which include Structural Text
(ST) and Instruction List (IL). IL consists of many step-by-
step lines of code where each line represents a single
operation. Due to its format and layout, entry of a series of
functions becomes easy. Also, being a low-level language,
enables it to execute faster as opposed to higher level
languages like Ladder. Additionally, IL is more compact and
its instructions and functions occupy small space in PLC
memory. However, it is not preferable due to the lack of visual
features making it hard to interpret and even more difficult to
trace errors. Therefore, IL is preferred in low-level processors.
Unlike IL, ST language is similar to high-level programming
languages. For instance, C and Pascal. Over the recent past,
ST has gained more attention due to its ease when it comes to
implementation of data analysis and calculus functions. It has
a flexible editor making insertion of comments and use of

indents to emphasize code sections much easier. Just like IL,
ST also executes faster than Ladder programs.
Task 5
The internal PLC architecture is shown in figure
Figure 1: PLC Internal Architecture [1].
The PLC has a CPU responsible for all control
operations. To fully execute its functions, the CPU has a
control unit to control timing operations, a memory unit for
storage, and ALU for arithmetic and data manipulation
functions. The buses are used for internal communication in
the PLC. Besides, the PLC has I/O unit which serves as the
interface that permits outside devices to interact with the
system [1].
References
[1] Bolton, W. (2009). Programmable logic controllers.
Amsterdam: Newnes.
[2] ECM. (2013, November 18). Understanding PLC
networks. Retrieved from
http://www.ecmweb.com/content/understanding-plc-
networks
[3] Gilbert, C. (2014, August 19). Advantages of a
Modular PLC Over a Fixed PLC? Retrieved from
https://blog.cetrain.isu.edu/blog/bid/353287/Advanta
ges-of-a-Modular-PLC-Over-a-Fixed-PLC
[4] Newark. (2018). Input Devices, Output Devices |
Automation | element14. Retrieved from
http://www.newark.com/input-devices-applications
[5] Ramanathan, R. (2014). The IEC 61131-3
programming languages features for industrial
control systems. 2014 World Automation Congress
(WAC). doi:10.1109/wac.2014.6936062