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This PLC reference is just a sample of what the textbook and workbook have to offer. Written by industry experts, this book covers important, up-to-date, real-world programmable controller topics and applications. This new edition is completely revised and updated to give you the latest developments and insights from the field. At 5 pounds and 1, pages, it puts all the PLC information you need at your fingertips.

And, since this is a generic PLC reference, it will help you with all of the different makes and models of PLCs in your facility. It was very easy to understand. And each of the review questions includes a detailed answer and explanation. The response time of any computer system must be fast enough to be useful for control; the required speed varying according to the nature of the process.

The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems for major U.


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Some companies proposed schemes based on the Digital Equipment Corp. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the because it was Bedford Associates' eighty-fourth project, was the result. One of the people who worked on that project was Dick Morley, who is considered to be the "father" of the PLC.

One of the very first models built is now on display at Modicon's headquarters in North Andover, Massachusetts.

It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range until the made its appearance. The automotive industry is still one of the largest users of PLCs.


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These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.

Many early PLCs did not have accompanying programming terminals that were capable of graphical representation of the logic, and so the logic was instead represented as a series of logic expressions in some version of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more common for ladder logic to be used, for the aforementioned reasons and because it was a familiar format used for electromechanical control panels.


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  6. Newer formats such as State Logic and Function Block which is similar to the way logic is depicted when using digital integrated logic circuits exist, but they are still not as popular as ladder logic. A primary reason for this is that PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer the person writing the logic to see any issues with the timing of the logic sequence more easily than would be possible in other formats. Early PLCs, up to the mids, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs [6].

    Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity.

    Beginner’s Free PLC Training Part 1 of 4: Introduction to PLCs - myPLCtraining

    The very oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on personal computers. The programming software allows entry and editing of the ladder-style logic. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking.

    The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. Regarding the practicality of these desktop computer based logic controllers, it is important to note that they have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than do PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs.

    In addition to the hardware limitations of desktop based logic, operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the logic may not always respond to changes in logic state or input status with the extreme consistency in timing as is expected from PLCs. Still, such desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical, because they are generally much less expensive than PLCs.

    In more recent years, small products called PLRs programmable logic relays , and also by similar names, have become more common and accepted. A typical processor usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing the results of the logical operations performed by microprocessor.

    The CPU controls all PLC activity and is designed so that the user can enter the desired program is relay ladder logic. The purpose of this interface is to condition the various signals received from or sent to external field devices.

    Why Use PLC Ladder Logic Examples?

    If the programming unit is not in use, it may be unplugged and removed. A program is a user- developed series of instructions that directs the PLC to execute actions. A programming language provides rules for combining the instructions so that they produce the desired actions. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced.

    Modern control systems still include relays, but these are rarely used for logic.

    Logic Gates vs Ladder Logic Circuits

    When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch or reed towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open non-conducting when the input is not energized.

    Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed conducting.

    PLC and Automation Training for Electricians

    An example of a relay in a simple control application is shown in Figure 3. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B.

    If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on. When we consider a PLC there are inputs, outputs, and the logic.

    PLC Basics

    Figure 4 shows a more complete representation of the PLC. Here there are two inputs from push buttons. This in turn drives an output relay that switches V AC that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact.

    Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types. The circuit shown in Figure 5 is an example of this, it is called a seal in circuit.