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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 electro-mechanical control panels. 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.
Programming[ edit ] PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. Often, a single PLC can be programmed to replace thousands of relays. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity. The oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on personal computers, which now represent the logic in graphic form instead of character symbols.
The programming software allows entry and editing of the ladder-style logic.
In some software packages, it is also possible to view and edit the program in function block diagrams, sequence flow charts and structured text. 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 software will upload and download the PLC program, for backup and restoration purposes. In some models of programmable controller, the program is transferred from a personal computer to the PLC through a programming board which writes the program into a removable chip such as an EPROM. The most commonly used programming language is Ladder diagram LD also known as Ladder logic. It uses Contact-Coil logic to make programs like an electrical control diagram.
A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. A model which emulated electromechanical control panel devices such as the contact and coils of relays which PLCs replaced.
This model remains common today. IEC currently defines five programming languages for programmable control systems: function block diagram FBD , ladder diagram LD , structured text ST; similar to the Pascal programming language , instruction list IL; similar to assembly language , and sequential function chart SFC.
Even within the same product line of a single manufacturer, different models may not be directly compatible.
Control example shown in ladder diagram[ edit ] This is a programming example in ladder diagram which shows the control system. A ladder diagram is a method of drawing control circuits which pre-dates PLCs.
The ladder diagram resembles the schematic diagram of a system built with electromechanical relays.
As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank by controlling the valve that refills the tank. Shown are: Two inputs from the low and high level switches represented by contacts of the float switches An output to the fill valve, labelled as the fill valve which it controls An "internal" contact, representing the output signal to the fill valve which is created in the program.
A logical control scheme created by the interconnection of these items in software In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system.
If a discrete input is energized, the memory bit is a 1, and a "normally open" contact controlled by that bit will pass a logic "true" signal on to the next element of the ladder. Therefore, the contacts in the PLC program that "read" or look at the physical switch contacts in this case must be "opposite" or open in order to return a TRUE for the closed physical switches. Internal status bits, corresponding to the state of discrete outputs, are also available to the program.
In the example, the physical state of the float switch contacts must be considered when choosing "normally open" or "normally closed" symbols in the ladder diagram.
Both float switches normally closed open their contacts when the water level in the tank is above the physical location of the switch. When the water level is below both switches, the float switch physical contacts are both closed, and a true logic 1 value is passed to the Fill Valve output. Water begins to fill the tank. The internal "Fill Valve" contact latches the circuit so that even when the "Low Level" contact opens as the water passes the lower switch , the fill valve remains on.
Since the High Level is also normally closed, water continues to flow as the water level remains between the two switch levels. Once the water level rises enough so that the "High Level" switch is off opened , the PLC will shut the inlet to stop the water from overflowing; this is an example of seal-in latching logic. The output is sealed in until a high level condition breaks the circuit.
After that the fill valve remains off until the level drops so low that the Low Level switch is activated, and the process repeats again. A complete program scan may take only a few milliseconds, much faster than changes in the controlled process. Programmable controllers vary in their capabilities for a "rung" of a ladder diagram. Some only allow a single output bit.
There are typically limits to the number of series contacts in line, and the number of branches that can be used. Each element of the rung is evaluated sequentially. If elements change their state during evaluation of a rung, hard-to-diagnose faults can be generated, although sometimes as above the technique is useful.
Some implementations forced evaluation from left-to-right as displayed and did not allow reverse flow of a logic signal in multi-branched rungs to affect the output. Functionality[ edit ] 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.
Desktop computer controllers have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than 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.
Operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the controller may not always respond to changes of input status with the consistency in timing expected from PLCs. 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.
Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of "examine if on" instructions will energize its output storage bit if all the input bits are on.
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It is intended for use in a mechanical domain, consequently.