Where is plc

The left and right rails indicate the positive and ground of a power supply. The rungs represent the wiring between the different components which in the case of a PLC are all in the virtual world of the CPU. So if you can understand how basic electrical circuits work then you can understand ladder logic.

In this simplest of examples a digital input like a button connected to the first position on the card when it is pressed turns on an output which energizes an indicator light. The CPU is then put into run mode so that it can start scanning the logic and controlling the outputs. Very good information. What is the differece between isolated output and isolated output? Also the arrangement of cards in the slots - is it okay to use any slots for input or output?

Also explain the meaning of the symbols used in the ladder diagram - like I, O and zero, one etc? Very good site to understand wat is PLC? I also want to ask to include in more specific details.

This site is wonderfull thank you for the information given it is usefull and clear, if you could explain solid state systems in more detail it would be helpfull. This website is a great help and it provides basic but very important information. It is exactly what I was looking for. Tools for PLC programming. Navigation Book reviews Online books Recent posts About. RSS Feed Me! Create new account Request new password. The components that make a PLC work can be divided into three core areas. Printer-friendly version.

PLC Submitted by sunnykmichael on Fri, Very good site to understand Submitted by Anonymous not verified on Sat, About site Submitted by S. Hi, i felt your website is very good for i can get nice answers for my Doubt. Bharani Dharan. I never believed that Submitted by Ote not verified on Thu, They are used in many industries to monitor and control production processes and building systems.

Once programmed, the PLC will perform a sequence of events triggered by stimuli referred to as inputs. It receives these stimuli through delayed actions such as counted occurrences or time delays.

These special computer devices are different from regular computers such as PCs or smartphones in that:. This microprocessor- based controller includes a programmable memory that stores instructions and implements functions that include sequencing, timing, logic, arithmetic, and counting.

This module is comprised of a central processor and its memory component. This processor performs all the needed data computations and processing by receiving inputs and producing corresponding outputs. This is essentially the module responsible for powering up the system. PLCs are used in various applications in industries such as the steel industry, automobile industry, chemical industry and the energy sector.

The scope of PLCs dramatically increases based on the development of all the various technologies where it is applied. In the Travel Industry, PLC has been used to monitor the safety control system and to operate lifts and escalators. PRCs controllers have been in use in the glass industry for decades. They are used largely to control the material ratio as well as to process flat glasses.

The technology has been advancing over the years and this has created an increased demand for the PLC control mode for use in the glass industry. The production of glass is an elaborate and sophisticated process so the companies involved often use PLCs with the bus technology in its control mode. Input devices can consist of digital or analog devices. A digital input card handles discrete devices which give a signal that is either on or off such as a pushbutton, limit switch, sensors or selector switches.

An analog input card converts a voltage or current e. Examples of analog devices are pressure transducers, flow meters and thermocouples for temperature readings. Output devices can also consist of digital or analog types.

A digital output card either turns a device on or off such as lights, LEDs, small motors, and relays. Typical outputs signals can range from VDC or mA and are used to drive mass flow controllers, pressure regulators and position controls. The most widely used form of programming is called ladder logic. Ladder logic uses symbols, instead of words, to emulate the real world relay logic control, which is a relic from the PLC's history.

These symbols are interconnected by lines to indicate the flow of current through relay like contacts and coils. Over the years the number of symbols has increased to provide a high level of functionality. The completed program looks like a ladder but in actuality it represents an electrical circuit. The left and right rails indicate the positive and ground of a power supply.

The rungs represent the wiring between the different components which in the case of a PLC are all in the virtual world of the CPU. So if you can understand how basic electrical circuits work then you can understand ladder logic. In this simplest of examples a digital input like a button connected to the first position on the card when it is pressed turns on an output which energizes an indicator light.

The CPU is then put into run mode so that it can start scanning the logic and controlling the outputs. The early history of the PLC is fascinating.

Imagine if you will a fifty foot long cabinet filled with relays whose function in life is to control a machine. Wires run in and out of the system as the relays click and clack to the logic. Now imagine there is a problem or a small design change and you have to figure it all out on paper and then shut down the machine, move some wires, add some relays, debug and do it all over again. Imagine the labor involved in the simplest of changes.

This is the problem that faced the engineers at the Hydra-matic division of GM motors in the late 's. Fortunately for them the prospect of computer control was rapidly becoming a reality for large corporations as themselves. So in the GM engineers developed a design criteria for a "standard machine controller". This early model simply had to replace relays but it also had to be:. By June of they were selling the first viable Programmable Controller the "" their 84th project which sold over one thousand units.

These early experiences gave birth to their next model the "" in which set Modicon as the early leader in programmable controllers. Not to be outdone, the powerhouse Allen-Bradley all ready known for it's rheostats, relays and motor controls purchased Information Instruments in and began development on this new technology.

By Odo Struger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin PLC which would make them successful for years to come. Allen-Bradley termed their new device the "Programmable Logic Controller" patent 3,, over the then accepted term " Programmable Controller ".

The PLC terminology became the industry standard especially when PC became associated with personal computers. At the heart of any computer system are the numbering systems and digital codes used for instructions and memory storage. Besides the ones and zeros it is important to understand how these bits are packaged into codes like BCD and Gray codes.

Understanding these concepts enables the programmer to manipulate the PLC at it's most basic level. Don't you want to feel the power at your fingertips!? Now for the most part these types of concepts will be transparent when programming but there will be occasions when you'll be glad you read this. When the cavemen first started counting they sat around looking at their fingers and stopped at ten. It took man a little longer to figure out the concept of zero but nobody is perfect.

So we've ended up with a common way of counting by 10's which in tech jargon is refered to as base or radix Base or radix refers to the number of symbols you have available to count.

We'll see that computers make it more difficult for us lazy humans because they like to count by base-2 binary. In a compromise with computers us humans have developed the base-8 octal or base hexadecimal systems.

If you don't get this joke then hopefully by the end of the chapter you'll get a chuckle out of it. Consider if you had no choice but to count with only a zero or a one. Pretend that the evil goblin of number snatchers had taken every number from two to nine.

How would you count anything? You'd have to do it like 0, 1, 10, 11, , , , and so on. You get the point? Therefore, if you had three apples then the third apple would be designated as 10 when in reality you only have three apples to eat. When it comes to computers and therefore PLCs they can only store in memory a 0 or a 1. That's the beauty of our digital age, it's either "on" or it's "off". Those memory chips in computers are actually made of rows upon rows of circuits that are either on at some voltage or off at some voltage.

Therefore a computer at it's very basic level can only count using a 0 off or a 1 on. That's why it's called binary because there are only two numbers like there are only two wheels on a bicycle. When you think about it, it's truly arbitrary how we count. You could also use an octal system by 8 or hexidecimal by 16 numbering system which we'll talk about a little later. Let's start off by looking closer at our all too familiar base 10 decimal system and then compare it to binary.

Decimal, like all these other number systems, is based on place-value system. This means that the value of a digit depends both on the digit itself and it's position within the number. The following figure shows the weights of a decimal number broken down into columns. The value of the number is computed by multiplying each digit by the weight of its position and adding up the results. Remember that for a base 10 system the weights are 1, 10, , and so on.

For a binary system the weights are 1, 2, 4, 8, 16, 32, 64, etc. What do you call a group of binary digits? The geeks who first thought this stuff up decided to call a binary digit a bit b -inary dig- it.

This is not to be confused with Tim-bits. After some time they decided that it would be good to call a group of 8 bits a byte. Funny bunch of geeks that they were the term nibble became used for 4 bits being a subset of byte.

Finally, a group of 16 bits are referred to as a word. Here's a picture to drive home the point. You're throbbing head is probably all ready telling you that binary numbers are not easy to read. As a compromise between humans and computers the octal base 8 and hexadecimal base 16 are used.

While octal is not as common as it's cousin hexadecimal it is still used in various PLCs so it's important to grasp the concept. Octal, like an octopus' eight legs, means eight and therefore there are eight numbers to use from zero to seven. The column weights are 1, 8, 64, , etc. Now we can do the same exercise as in the last chapter to convert an octal number to decimal. I know this isn't helpful so far. Where it really comes in handy is coverting from binary to octal because all you have to do is break down the binary number into chunks of three.

This is because 8 is 2 3. Most programmable controllers have inputs and output cards grouped in 8 or 16 and high density of 32 and The reason for this is the way computers like to have things in powers of 2, 4, 8, 16 and so on. So if it is not in octal it is typically in hexadecimal. Hexadecimal is a little more tricky because it is base 16 and therefore we need something beyond 0 through 9 for symbols and this is done by using the letters 'A' through 'F'.

Hexadecimal is used for the same reasons as octal so that we can represent binary in a condensed form and make it easier for conversion. Where octal used 3 bits the hexadecimal system used 4 bits to represent one number. If you're going to be programming something like a Mitsubishi PLC then you better get used to hexadecimal. To wrap things up here's a table below showing the equivalents for each numbering system. Decimal Binary People: Learning your 1s and 0s. Let's delve deeper into PLC programming by considering again our common word made of 16 bits.

If it was all filled up with one's then the decimal value would be So a range from 0 to could be represented.

Adding binary numbers together would be very similar to addition in decimal. The problem comes when you need to subtract. How do you represent a negative number when you can't just put a minus sign in front of it and say it's good? Remember that the computer can only do a 0 or a 1. To our rescue comes a concept called taking the complement.

Complement 's are a pretty cool trick and you can learn more about them at Wikipedia. We'll keep it simple here and talk about two's complement which is the most common in computers and PLCs.

Signed binary numbers are achieved by stealing the 16th bit in a word the most significant bit and using that as a sign bit where 0 is positive and 1 is negative. So the high end of our value is decreased but we've made it a whole lot easier to indicate a negative number and do subtraction. Here's how it works. Let's take a number like 30 and perform the two's complement to get The number 30 is The first step is to invert or flip the bits The second step is to add one In the PLC then the value of will be respresented as Maybe not what you would expect?

The beauty of this is that now all the processor has to worry about is adding the two numbers to get the correct value. See how this magically works in the table below I'm just going to use 8 bits now to simplify things but it works just the same with however many bits you want. Decimal Signed Binary People: Learning your 1s and 0s. For the most part this will all work seamlessly in the background while you program away. It's just that every once in awhile you'll need this knowledge to overcome any limitations in the system.

Dealing with negative numbers can be pretty tricky but with complements like this you better not let it go to your head. Floating point numbers also known as 'real numbers' give a certain freedom in being able to represent both very large and very small numbers in the confines of a 32 bit word that's a double word in our PLCs. Up until this point the range of numbers we were able to represent with a double word would be from 0 to 4,,, Floating point on the other hand allows a representation of 0.

It allows for such large numbers that we can even keep track of the US national debt. Floating point gives us an easy way to deal with fractions. Before, a word could only represent an integer, that is, a whole number.