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Features

Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, relay units for in- and output

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. These connect the PLC to sensors and actuators. PLCs read 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, PLCs 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.

[edit] System scale

A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O.

Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.

[edit] User interface

See also: User interface

See also: List of human-computer interaction topics

PLCs 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. HMIs are also referred to as MMIs (Man Machine Interface) and GUI (Graphical User Interface).

A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use a programming and monitoring software installed on a computer, with the PLC connected via a communication interface.

[edit] Communications

PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Other 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 PLCs 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.

[edit] Programming

PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLCs replaced. This model remains common today.

IEC 61131-3 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.

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PLC & input/output arrangements

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. PLCs are used in many industries and 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.

راهنمای استفاده از کنتور سه تعرفه دیجیتال

- در
کنتورهای مکانیکی تعین میزان مصرف برق از طریق نمراتور کنتور وبا کسر عدد فعلی از قبلی به دست می آید و مصرف به دست
آمده را کلا مطابق با یک نرخ (ساعات میان باری روز )محاسبه
وبهای برق مشترک بدست میاید :

به طور مثال اگر مشترک در24ساعت شبانه روزی 20کیلو وات
ساعت  برق مصرف کند (با فرض هر کیلو وات ساعت 200 ریال) مبلغ (400=20×20)
ریال می شود

                     (کنتور مکانیکى)

2- در کنتور دیجیتالی سه تعرفه ،مصرف در 24ساعت شبانه روز
به سه بازه زمانی شامل ساعات میان باری (روز) ،ساعات اوج بار(شب) ،وساعات کم باری
(آخر شب) تقسیم شده ونرخ هر کدام متفاوت وبه ترتیب با ضرایب 1، 5/2، 25 /0نسبت به
نرخ ساعات میان باری مساحبه می گردد

             (کنتور دیجیتال)

 ساعات بازه زمانی در6ماه اول ودوم سال در جدول
زیر می باشد

به طور مثال اگر یک مشترک در 24 ساعت شبانه روز 20کیلو وات
ساعت برق مصرف کند ودارای کنتور دیجیتالی باشد کنتور مصرف برق فوق را فرضا به سه
بخش تقسیم می کند

 مصرف ساعات میان باری (روز ) =10کیلو وات ساعت

مصرف ساعات اوج بار (اول شب ) =  3کیلو وات ساعت

مصرف ساعات کم باری (آخر شب ) =  7 کیلو وات ساعت

ضریب×نرخ×مصرف = بهای برق مصرفی با
کنتور دیجیتال

3850= (25/0×200×7) +(5/2×200×3) + (1×200×10) ریال

بهای برق مصرفی این مشترک معادل 3850ریال در یک شبانه روز
می باشد

براى
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