Definations of PLC Programmable logical controller.

A specialized device used to provide high-speed, low-level control of a process. It is programmed using LD or some form of structured language, so that engineers can program it. PLC hardware typically costs a lot of money, and often has very good redundancy and fail-over capabilities. When you walk around a factory, the thing with all the lights and buttons and switches is the PLC (or possibly a DCS, which is a big PLC).

(Programmable Logic Controller) - An electronic control unit used to regulate the operation of a given packaging machine. PLCs control the machine’s operating parameters and allow programming or changing of those parameters to accommodate differing application requirements.

microprocessor-based industrial control system. It communicates with other process control components through data links. It is used in process control for simple switching tasks, PID control, complex data manipulation, arithmetic operations, timing and process and machine control.

Digital vs. Analog Signals

Digital or discrete signals behave as switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Pushbuttons, limit switches, and photo-eyes are examples of devices providing a digital signal. Digital signals are judged using either voltage or current, where a specific range is denominated as On and another as Off. A PLC might use 24VDC I/O, with values above 22VDC representing On and values below 2VDC representing Off. Initially, PLCs had only digital I/O.

Analog signals behave as volume controls, yielding a range of values between zero and full-scale. These are typically interpreted as integer values by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. Pressure, temperature and weight are examples of measurements which can provide analog signals. Analog signals can use voltage or current, but do not have discrete ranges for On or Off. Instead they work in a defined a range of valid values where the I/O device can operate reliably. On a 0-24VDC scale, 25VDC may be interpreted by the PLC as any value.

PLCs programs are generally written in a special application on a personal computer then downloaded over a custom cable to the PLC. The program is typically stored in the PLC either in battery-backed-up RAM or some other non-volatile memory.

Early PLCs were designed to be used by electricians who would learn PLC programming on the job. These PLC's were programmed in "ladder logic", which strongly resembles a schematic of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams.