grmili.blogg.se

The lower-left screw terminal is a “Common” connection, which is generally connected to L2 (neutral) of the 120 VAC power source.

Six screw terminals on the left-hand side provide connection to input devices, each terminal representing a different input “channel” with its own “X” label. Two screw terminals provide connection to 120 volts AC for powering the PLC’s internal circuitry, labeled L1 and L2. The following illustration shows a simple PLC, as it might appear from a front view. Signal connection and programming standards vary somewhat between different models of PLC, but they are similar enough to allow a “generic” introduction to PLC programming here. Although some PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in logic gate circuits, this is the exception and not the rule. PLCs are industrial computers, and as such their input and output signals are typically 120 volts AC, just like the electromechanical control relays they were designed to replace. Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions. In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams.

It also has many output terminals, through which it outputs “high” and “low” signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions.Ī PLC has many “input” terminals, through which it interprets “high” and “low” logical states from sensors and switches. Systems and processes requiring “on/off” control abound in modern commerce and industry, but such control systems are rarely built from either electromechanical relays or discrete logic gates. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching. Remote Monitoring and Control of PLCs Via Digital Computer Networksīefore the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays.