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# Disadvantages of Programmable logic controller (PLC, programmable controller) control

:-

1. There's too much work required in connecting wires.

2. There's difficulty with changes or replacements.

3. It's always difficult to find errors; And require skillful work force.4. When a problem occurs, hold-up time is indefinite, usually long.

# Advantages of Programmable logic controller (PLC) control :-

1. Rugged and designed to withstand vibrations, temperature, humidity, and noise.2. Have interfacing for inputs and outputs already inside the controller.

3. PLCs are easily programmed and have an easily understood programming language.

Circui

t Diagram

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A Circuit Diagram is the key!

How can a circuit diagram be a key, it's just a load of funny squiggles isn't it?Yes it is to the non-technical person, but to a technical person, a circuit or schematic diagram is

very much a key to the equipment concerned. You could call the schematic diagram the coded

key to the pulse of life in any piece of equipment, big or small.

Squiggles they may be but without it, to those who understand how to read them, repairing

equipment can be very much more difficult, if not nearly impossible.

A circuit diagram also called an electronic or electrical diagram, wiring schematic or electronicschematic is a simplified graphical representation of an electrical and/or circuit.

They show the actual components that make up the circuit as a set of simplified graphic type

symbols, also it shows the signal and power connections between the components.

The circuit diagram above is quite a typical PLC circuit arrangement. This PLC is powered by24V DC, hence the PSU in the top left corner, F1 (Fuse) is the mains fuse and is there to protectthe PSU and wiring supplying it. F2 (2A) is the 24V DC fuse and is there to protect the wiring

and the PLC, you'll notice this line connects to the 24V terminal on the PLC.

You'll also notice the 0V connection on the PLC, this is the return line or common. Conventional

electron (electricity) flow is from positive (+24V) to negative (0V or Common). So followingthis power going out through switches etc returnson the 0V or common line, generally depicted

as 'Com'.

The arrangement of these components within the diagram does not correspond to their physical

dimensions or layout arrangement on the printed circuit board (PCB) or internal arrangement ofthe finished device. And it is not meant to.

A schematic diagram differs from a block diagram or overview diagram in as much that it is a

diagram of the actual wired connections of the circuit being described. A circuit drawing is

meant to show the actual physical wired connections and the components of the electronic"physical design".

Circuit schematic diagrams are also used to generate the PCB design and lend alot to the

electrical layout and construction of a circuit. They are also invaluable for the fault finding and

maintenance of electrical and electronic equipment.

But, to read them is like learning a new language, but even for the fluent in electronic speak,

sometimes they're not always easy to follow. I know what you're probably thinking, why are they

drawn like that. Is it deliberate to confuse people, or is there a real reason.

You know what! There is a very real reason and basically it's done to make a universalstandardof electrical, electronic, mechanical, pneumatic and hydraulic symbols for drawing circuit

diagrams.

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The electrical diagram or electronic schematic is a method of drawing a simplified representation

of an electrical or electronic circuit of the equipment in question.

The reason to standardise the the symbols is to aid description/recognition of how the equipment

operates, or is supposed to operate in the case of fault finding.

Although electrical and electronic circuit symbols have changed over the years, they also differ

from country to country. Not differing so much that they would be undefinable though, they have

remained basically the same, just some subtle changes.

Much the same as the differences from country to country using the samestandard. There is astandard that most companies follow and follow well. The drawing of components within the

standard does change a little from country to country but with a little exposure to electronics this

should not be of great concern.

The circuit diagram shows the signals and power connections between all the individual

components involved in the operation of any given piece of equipment. Most circuit diagramsare drawn following thepowerflow.

This means going from the power supply, to the higher power components, then the lower power

components and following that, switches and sensors involved in the operation of the equipment.Then after that, things like volt free contacts giving condition indications to other equipment that

may be driven or monitoring.

The schematic does not generally give any real indication as to the physical layout of the

individual component parts within it'sgutsor under the covers. But this is not always the case,

sometimes it amkes sense to be drawn like that. The way the equipment operates hardly ever

relates to its physical layout.

Circuit diagrams are primarily for the design and maintenance of electronic, electrical, pneumaticand hydraulic equipment. Electro-mechanical parts are quite often used in machinery and the like

and is more prone to failure than just electronic based equipment.

To this ends a circuit is invaluable to the engineer or technician undertaking it's fault finding and

repair,yesI'm speaking from a great number of years of experience here.

Symbol standards used are either the ANSI standard Y32 or the IEC standard 617. Different

symbols are often used depending on the subject of the actual drawing concerned. For an

example, lighting and power symbols used in an architects drawings may well be different fromsymbols for similar devices used in electronics.

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