The best PLC Tutorial
on the Internet!This electrifying PLC tutorial will tell you all you need to know about PLC’s. Its job is to introduce the PLC to any PLC virgin’s that may be looking and those whom know a little bit about ‘em, but want know more about the in the PLC arena. Further more, I believe it’s the best PLC tutorial on the Internet, go on see if you don’t agree and read on and judge for yourself!"Where did they come from?" "What the origins of the PLC?" "What does PLC mean?" Ok, ok, one question at a time! Let me tell you a story, are you sitting comfortably?
This is a Finder 4 pole change over relay with a 24V AC coil, on a blue base. Oh look, here’s a relay! Before PLC’s (and this PLC tutorial, honest) the only way to make up a control circuit for a machine was with. . . Yep relays, as you can imagine on a machine with many motors and solenoids (posh word for electromagnet) to control, this took many relays. All of which had to be wired together in the right order. I’m glad I didn’t get that job then! This style of control panel gave rise to several problems, as the relays were mechanical they failed often. Coil failure and contact wear were difficult to diagnose and replace, also if a circuit change was needed that called for a rewired of all the involved relays. A panel with dozens of relays consumed oodles of power. So one day a smart bloke called Richard E. Morley, an employee of Bedford Associates, invented the first Programmable Logic Controller (Aha PLC) as a consultation project for the General Electric Company in 1968. Bedford Associates changed their name after some time to Modicon PLC and became a supplier of PLC’s. The commercial version didn’t come along until several years later of course, they didn’t have a PLC tutorial in those days of course, so Mr Morley had to make the PLC easy to understand and program for the technicians of the day, so the relay Ladder Logic, as it’s called, style of operation was copied into the PLC software. Now would you be surprised to know Ladder Logic is still by far the most popular way to program PLC’s today, of course not. Nowadays PLC’s are programmed via a desk computer or laptop, but back then computers weren’t around. The programming of these ingenious devices had to be done with something call a handheld programmer. These weren’t actually always handheld, some of them would be clipped onto the front of the PLC itself to program the PLC.
Just like this one here, although it's not that old (only about 20 years) it is very rare now and cannot be purchased anymore through the normal channels. But I managed to get hold of it just for you, to take a picture for this PLC tutorial. (No expense spared for this PLC tutorial you see) This particular programmer is a Mitsubishi F1-20P and to use it you would be entering one relay switch or coil at a time. Quite laborious as you can imagine, especially if your prospective proggy is upwards of about 50 steps long (one step is a relay switch or coil). I'm glad I wasn't programming then, as generally most programs would be! Ok, so what does a Ladder Logic program look like then? Well something like this!
This little snippet of ladder program is 17 steps in size, quite small really and it doesn’t do much, but that doesn’t matter as it’s a PLC tutorial example! Even so, it has four inputs (X0 to X3 remember) and two outputs (Y0 and Y1), two internal relays (M0 and M1), also two timers (T0 and T1) and a counter (C0) lurking around in there somewhere. I expect you can easily see why it's called Ladder Logic' now, yep because it look like a ladder. You may also notice how it compares to a relay control circuit diagram. Working from left to right, the supply (+24V DC, 110V AC) would be the vertical bar on the left, followed by the switches, then the relay coils, which connect to the ground (0V or common) return supply, the vertical line on the right.
In a PLC program, the 'X' switches would be the only physical inputs, the other switches and coils would be in the plc's memory, apart from the 'Y' coils which would be the only physical outputs. If you had a closer look at the picture on the very top of this page, you can probably make out the 'Y' numbers on the bottom row of terminals. The 'X' numbers are the top row of terminals, but are slightly obscured by the terminal cover, but they are there. But don't strain your eyes, here they are and I've lifted the terminal covers so you can see them more clearly. If you look very closely at the numbers you can see they go from X17 to X20 and X27 to X30. What! Can't they count then! Yes, I'm sure they can because the reason behind this numbering mistake is down to how computer data lines are wired.
The PLC computer data lines are wired in multiples of eight, for instance, 'Windows XP 32bit' is operating on four sets of data bus lines. But data lines aren’t numbered one to eight, oh no it's zero to seven, sooo that's...Yep This is true not just of PLC's in this PLC tutorial, but for all sorts of computers. These days PLC’s form the central hub of many manufacturing processes. These microprocessor based boxes of electronicals are used everywhere in industry from simple machine projects such as one operation box packing machines to the tracking and control of very highly sophisticated, fully automated warehouse systems. You will find them in virtually all new and most existing manufacturing, processing, printing and packaging machine equipment. So it won’t surprise you for me to tell you in this PLC tutorial, that because of their popularity being so widespread in industry, it has become increasingly more and more important for technicians to learn the skills needed to use and find fault with these wonderful devices. Ok so now let’s dig a bit further into the guts of a PLC, you’ve come this far I’m sure you’d like to know some more about what make a PLC tick! Speaking of ticking, the bit in all of these little beasties that does the ticking is the Microprocessor. The microprocessor or the processor module is the brain in the box, but nowadays microprocessors are a far cry from the 1960’s microprocessors. There is a microprocessor bringing this PLC tutorial to your screen of course. In essence the job of the microprocessor is to monitor the condition or state of the inputs, use this data in performing the logic of the user program and then update the state of the outputs accordingly. Some PLC’s only have as little as 6 inputs and outputs (referred to as I/O) in total to control, but others can have in the order of forty thousand or more I/O. One single processor is capable of controlling just one process or a whole manufacturing empire, internationally. Quite an easy job when there’s only 6, but when there’s around forty thousand . . . WOW!!! That’s a fast processor. So it must have somewhere to store the data that represents the inputs status, well yes it does, in memory called RAM. Also you’ll find some ROM in a PLC, this is Read Only Memory, this is the opposite of RAM, as it’s called a non-volatile memory and it doesn’t lose its data if power is removed. The information held in ROM is burnt into it by a special process and normally contains the operating system of the PLC. Also as the name suggests, the data can only be read from it, not written to it or changed. Then there will be an amount of EEPROM, this abbreviation means Electrically Erasable Programmable Read Only Memory, aha yes that’s why they’ve abbreviated it then! I’ve forgotten what this memory does . . . It’s a good job we haven’t run out of memory for this PLC tutorial, otherwise you wouldn’t be reading all this useful info now. Now what else would you like to know as there is much more to see in addition to this tutorial from the PLC page. Return from PLC Tutorial to the PLC page. Go from here to the Technical page. Return from PLC Tutorial to Home.
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