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.

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.

Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens and so forth.

To implement such controllers electrical engineers may use electrical circuits, digital signal processors, microcontrollers and PLCs ( Programmable Logic Controllers ).

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.

However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomical.

These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays.

The large library of special blocks along with high speed execution has allowed use of PLCs to implement very complex automation systems.

The notation of ladder logic is still in use as a programming idiom for PLCs.

The IEC 61131 programming tool is more popular for use with PLCs, while RTUs often use proprietary programming tools.

Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers ( PLCs ).

Given that PLCs usually control systems which are easily represented by a state transition diagram, the use of a very high-level programming language such as state logic greatly helps the PLC programmer in making intuitive control programs.

PLCs used in larger I / O systems may have peer-to-peer ( P2P ) communication between processors.

PLCs are used in many industries and machines.

More recently, PLCs are programmed using application software on personal computers.

The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers.

These are very much like PLCs, and are used in light industry where only a few points of I / O ( i. e. a few signals coming in from the real world and a few going out ) are involved, and low cost is desired.

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 logic.

The main difference from other computers is that PLCs are armored for severe conditions ( such as dust, moisture, heat, cold ) and have the facility for extensive input / output ( I / O ) arrangements.

Modular PLCs have a chassis ( also called a rack ) into which are placed modules with different functions.

PLCs are well adapted to a range of automation tasks.

RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa.

Additionally, modern industrial automation equipment, such as PLCs, VFDs, servo drives, and CNC equipment are programmable via RS-232.

They are activated by AC control signals or DC control signals from Programmable logic controller ( PLCs ), PCs, Transistor-transistor logic ( TTL ) sources, or other microprocessor and microcontroller controls.

To control the parameters in a process or in a particular system, devices such as microprocessors, microcontrollers or PLCs are used, but their ultimate aim is to control the parameters of a system.

* Programmable logic controller ( PLCs ) used as field devices because they are more economical, versatile, flexible, and configurable than special-purpose RTUs.

Most control actions are performed automatically by RTUs or by PLCs.

Use of " smart " RTUs or PLCs, which are capable of autonomously executing simple logic processes without involving the master computer, is increasing.

PLCs are specialized hardened computers which are frequently used to synchronize the flow of inputs from ( physical ) sensors and events with the flow of outputs to actuators and events.

Today, most such systems are constructed with programmable logic controllers ( PLCs ) or microcontrollers.

In a commercial setting, watering frequency is multi factorial and governed by computers or PLCs.

Among the systems which benefited from having a 68000 or derivative as their microprocessor were families of Programmable Logic Controllers ( PLCs ) manufactured by Allen-Bradley, Texas Instruments and subsequently, following the acquisition of that division of TI, by Siemens.

The automotive industry is still one of the largest users of PLCs.

Early PLCs were designed to replace relay logic systems.

These PLCs were programmed in " ladder logic ", which strongly resembles a schematic diagram of relay logic.

Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.

Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C. Another method is State Logic, a very high-level programming language designed to program PLCs based on state transition diagrams.

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.

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 mid-1980s, 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.

The very oldest PLCs used non-volatile magnetic core memory.

PLC-like programming combined with remote I / O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.

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.