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PLCs (Programmable Logic Controllers) are devices that control plant or machine operation based on defined, programmed sequences and equipment status. A PLC is an electronic device which may be part of a standalone unit with on-boad I/O, or a single (but vital) part of a much larger, modular system. A PLC communicates with devices across plant and makes decisions accordingly. There are two basic types of interface with generic plant equipment: Digital I/O and Analogue I/O. Digital I/O is a discrete signal (ON or OFF) that is either read from a device (such as a motor indicating running or not running), or a discrete command that is sent to a device (such as the motor being told to run or not, or an alarm light being on or off). Analogue I/O is a continuous signal between defined limits that provides highly accurate information to or from a device. An example of an Analogue Input would be a water level in a tank, read from a level transmitter, while an analogue Output could be a setpoint sent to a variable speed drive to control the speed of the pump taking water from the tank. Most plants will have a combination of digital and analogue I/O, with a further variation amongst the actual types used, such as different voltages or currents. I/O in modular systems may be located all in a single electrical cabinet, or distributed across a machine or area, with some type of bus system used to communicate data to/from the PLC. A PLC may then form part of a larger, distribulted system of one or more PLCs that can communicate with each other and also the HMI/SCADA system(s). These communications networks may vary from hard wired to wireless, from simple to more complex or redundant, and from propietary to open standards. There are quite a few PLC manufacturers offering a multitude of different products and communications standards. Downey Engineering have the experience, software and tools to deal with many. We are proud to be integrators for Allen-Bradley, Schneider and Siemens and to offer supply,
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