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.
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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.
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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 may contain thousands of rungs, evaluated in sequence. 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. 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.
The most basic function of a programmable controller is to emulate the functions of electro-mechanical relays. 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.
Similarly, a parallel set of instructions will perform a logical OR. In an electro-mechanical relay wiring diagram, a group of contacts controlling one coil is called a "rung" of a "ladder diagram ", and this concept is also used to describe PLC logic.
Some models of PLC limit the number of series and parallel instructions in one "rung" of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an "internal coil" with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs.
Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program. Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which in a sufficiently complex circuit may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these "sneak paths" is either a bug or a feature, depending on programming style.
More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variable internally that may not correspond to discrete logic. The main function of a timer is to keep an output on for a specific length of time. A good example of this is a garage light, where you want power to be cut off after 2 minutes so as to give someone time to go into the house.
A Delay-OFF timer activates immediately when turned on, counts down from a programmed time before cutting off, and is cleared when the enabling input is off.
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A Delay-ON timer is activated by input and starts accumulating time, counts up to a programmed time before cutting off, and is cleared when the enabling input is turned off. A Delay-ON-Retentive timer is activated by input and starts accumulating time, retains the accumulated value even if the ladder-logic rung goes false, and can be reset only by a RESET contact. Counters are primarily used for counting items such as cans going into a box on an assembly line.
This is important because once something is filled to its max the item needs to be moved on so something else can be filled.
Many companies use counters in PLC's to count boxes, count how many feet of something is covered, or to count how many pallets are on a truck. Up counters count up to the preset value, turn on the CTU CounT Up output when the preset value is reached, and are cleared upon receiving a reset. Down counters count down from a preset value, turns on the CTD CounT Down output when 0 is reached, and are cleared upon reset. In more recent years, small products called PLRs programmable logic relays , and also by similar names, have become more common and accepted.
These small devices are typically made in a common physical size and shape by several manufacturers, and branded by the makers of larger PLCs to fill out their low end product range. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Size is usually about 4" wide, 3" high, and 3" deep. Most have a small plug for connecting via RS or RS to a personal computer so that programmers can use simple Windows applications for programming instead of being forced to use the tiny LCD and push-button set for this purpose.
Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC, and they still offer robust design and deterministic execution of the logics. PLC input can include simple digital elements such as limit switches , analog variables from process sensors such as temperature and pressure , and more complex data such as that from positioning or machine vision systems.
A PLC program generally loops i. The program then runs from its first instruction rung down to the last rung. Excessively long scan times may mean the response of the PLC to changing inputs or process conditions is too slow to be useful. As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented.
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This means that IO is updated in the background and the logic reads and writes values as required during the logic scanning. Precision timing modules, or counter modules for use with shaft encoders , are used where the scan time would be too long to reliably count pulses or detect the sense of rotation of an encoder. This allows even a relatively slow PLC to still interpret the counted values to control a machine, as the accumulation of pulses is done by a dedicated module that is unaffected by the speed of program execution on the PLC.
A small PLC will have a fixed number of connections built in for inputs and outputs.
Automated Manufacturing Systems; PLCs - Lab Guide
Modular PLCs have a chassis also called a rack into which are placed modules with different functions. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface HMI is employed for this purpose. A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.
Various protocols are usually included.
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Automated Manufacturing Systems; PLCs - Lab Guide
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