【文章內(nèi)容簡(jiǎn)介】 ), flow chart, C, and Basic. The IEC 611313 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and Instruction List. This allows for multivendor patibility and multilanguage programming. SFC is a graphical language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences. FBD uses a broad function library to build plex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable munication and interface functions. LD is a graphic language for discrete control and interlocking logic. It is pletely patible with FBD for discrete function control. ST is a text language used for plex mathematical procedures and calculations less well suited to graphical languages. IL is a lowlevel 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 for programmable logic controllers (PLC). 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 function libraries. C is a high level programming language suited to handle the most plex putation, 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. Programmable logic controllers can also be specified with a number of puter interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider.2. INTRODUCTIONFor simple programming the relay model of the PLC is sufficient. As more plex functions are used the more plex VonNeuman model of the PLC must be used. A VonNeuman puter processes one instruction at a time. Most puters operate this way, although they appear to be doing many things at once. Consider the puter ponents shown in Figure 1.Figure 1 Simplified Personal Computer ArchitectureInput is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outputs.Figure 2 An InputOutput Oriented ArchitectureIn this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal puter as a controller, it is controlling the user by outputting stimuli on the screen, and inputting responses from the mouse and the keyboard.A PLC is also a puter controlling a process. When fully integrated into an application the analogies bee。Inputs the keyboard is analogous to a proximity switch.Input circuits the serial input chip is like a 24Vdc input card.Computer the 686 CPU is like a PLC CPU unit.Output circuits a graphics card is like a triac output card.Outputs a monitor is like a light.Storage memory in PLC is similar to memories in personal puters.It is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and rugged for harsh production environments. 3. OPERATION SEQUENCEAll PLC have four basic stages of operations that are repeated many times per second. Initially when turned on the first time it will check its own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the logic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs will be updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults. This process typically repeats 10 to 100 times per second as is shown in Figure 3.Figure 3 PLC Scan CycleSELF TEST Checks to see if all cards error free, reset watchdog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time this would indicate that the ladder logic is not being scanned normally).INPUT SCAN Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables.LOGIC SOLVE/SCAN Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.OUTPUT SCAN The output table is copied from memory to the output chips. These chips then drive the output devices.The input and output scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inputs, and solves the logic. This prevents potential problems that might occur if an input that is used in multiple places in the ladder logic