1、 1 / 16 PLC 1 .About Programmable Logic Controllers (PLCs) PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control eq
2、uipment. They are programmable via software interfaced via standard computer interfaces and proprietary languages and network options. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of chan
3、nels. Number of points is the sum of the inputs and the outputs. PLCs may be specified by any possible combination of these values. Expandable units may be stacked or linked together to increase total control capacity. Maximum number of channels refers to the maximum total number of input and output
4、 channels in an expanded system. PLC system specifications to consider include scan time, number of instructions, data memory, and program memory. Scan time is the time required by the PLC to check the states of its inputs and outputs. Instructions are standard operations (such as math functions) av
5、ailable to PLC software. Data memory is the capacity for data storage. Program memory is the capacity for control software. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLCs include DC
6、, AC, relay, analog, frequency or pulse, Programming options for PLCs include front panel, hand held, and computer. Programmable logic controllers use a variety of software programming languages for control. These include IEC 61131-3, sequential function chart (SFC), function block diagram (FBD), la
7、dder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic. The IEC 61131-3 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structu
8、red Text, and Instruction List. This allows for multi-vendor compatibility and multi-language programming. SFC is a graphical language that provides coordination of program 2 / 16 sequences, supporting alternative sequence selections and parallel sequences. FBD uses a broad function library to build
9、 complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable communication and interface functions. LD is a graphic language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control. ST is a t
10、ext language used for complex mathematical procedures and calculations less well suited to graphical languages. IL is a low-level language similar to assembly code. It is used in relatively simple logic instructions. Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language f
11、or programmable logic controllers (PLCs). Ladder logic programming is a graphical representation of the program designed to look like relay logic. Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reusable fu
12、nction libraries. C is a high level programming language suited to handle the most complex computation, sequential, and data logging tasks. It is typically developed and debugged on a PC. BASIC is a high level language used to handle mathematical, sequential, data capturing and interface functions.
13、Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider. 2. PLC hardware PLC hardware mainly has the central pr
14、ocessing unit (CPU), memory, input unit, output unit, the communication interface, expansion interface power components. Among them, the CPU is the core, input unit of PLC with output unit is connected site input/output devices to the CPU, interface circuit between the communication interface used w
15、ith programmer, connecting peripherals such as the PC. 3 / 16 2.1 The central processing unit (CPU) The same sort of microcomputer is same, the CPU is the core of PLC. PLC deployed by the CPU in different and different, with models used into three categories: general microprocessor (such as 8086, Z8
16、0, 80286, etc.), single chip microprocessor (such as 8031, etc) and a piece 8096 microprocessors (such as AMD29W, etc.). Small PLC is used mostly eight general microprocessor and single chip microprocessor； Medium PLC is used mostly 16 general microprocessor or single chip microprocessor； Large PLC
17、mostly by using high-speed a piece microprocessors. 2.2 memory Memory basically has two kinds: one kind is read/write operation of random access memory RAM, another kind is read only memory (ROM, PROM, an EPROM and EEPROM. In PLC, memory is mainly used for storage system programming, the user progra
18、m and job data. System programming is written by PLC manufacturer, and PLC hardware related, complete system diagnosis, command interpretation, function subroutine calling, management, logic operations, communications and various parameters, and other functions, provide PLC operation platform. Syste
19、m programming related to the performance, but also in PLC PLC in use process fluctuant, so is not directly by manufacturer in read-only memory ROM, curing PROM or an EPROM, users cant access and modification. The user program with PLC control object is determined by users, according to the object of production process control requirements and prepare application. In order
