Контрольные вопросы:

1. What is a computer networking?

2. What does computer networks rely upon?

3. What are all networks interconnected for?

4. Who are interconnected by Internet Service Providers (ISP)?

5. What do you know about a local area network?

Text VII programmable logic controller

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides or lighting fixtures, PLC’s are used in many industries and machines, such as packaging and semiconductor machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.

Features. The main difference from other computers is that PLC’s are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLC’s real limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision. On the actuator side, PLC’s operate electric motors. Pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

PLC’s 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. HMI’s are also referred to as MMI’s (man Machine Interface) and GUI (Graphical User Interface).

PLC’s have built in communications ports usually 9-Pin RS232, and optionally for RS485 and Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Others’ options include various fieldbuses such as DeviceNet or Profibus. Other communications protocols that may be used are listed in the List of automation protocols.

Most modern PLC’s can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.

PLCS used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations. Some of today’s PLC’s can communicate over a wide range of media including RS-485, Coaxial, and even Ethernet.