1、 外文资料翻译 PLC THEORY AND COMPOSITION I Principle of work 1.Scanning works When the PLC is running, through the implementation of the reflecting control requirements of the user program to complete the control task, the need to perform numerous operations, but the CPU can not simultaneously to perform
2、multiple operations, it is only by time-sharing operation (serial work) way Each time an action is performed, sequentially, the Executive. As the CPUs processing speed is fast enough that from a macro point of view, PLC exterior appearance of the results seem to be at the same time (parallel) comple
3、tion. This serial work process is known as PLCs scanning work. With a scanning manner the user program, the scan is from the first program started, without interruption or jump under the control, according to the procedure stored in the order of the sequence, one by one implementation of the user pr
4、ogram until the program ends. And then start scanning the implementation of the cycle to repeat run. PLC scanning work and electrical control works significantly different. Electrical control devices using hard logic of parallel work, if a relay coils or power, then the relay all the normally open,
5、and normally closed contact, whether in a control circuit which position, will immediately and simultaneously move ； the PLC using the scanning mode (serial works), if a soft relay coil is connected or disconnected, all of its contacts will not immediately move, have to wait for scan to the time bef
6、ore action. However, PLCs scan rate, typically PLC and electrical control devices in the I / O processing results on no difference. 2 .PLC scanning process PLC scanning work process in addition to the implementation of user procedures, in each scanning process must complete the internal processing,
7、communications services. The whole scanning process including internal processing, communications services, input sampling, program execution, the output refresh the five stages. The whole process scan execution once the time called the scanning cycle. Scanning cycle and the CPU speed, PLC hardware
8、configuration and user program the length of the typical value of 1 100ms. In the internal processing stage, for PLC self-test to check the internal hardware is normal, and watchdog timer (WDT) reset and the completion of some other internal processing. In the communication service stage, PLC and ot
9、her intelligent devices for communication, respond to programmer type command, update the programmer display content. When the PLC is in stop (STOP) state, the only complete internal processing and communication services. When the PLC is running (RUN) state, in addition to the completion of internal
10、 processing and communication services, but also the completion of input sampling, program execution, the output refresh work.PLC scanning work is simple and intuitive, easy to program design, and reliable operation provided a guarantee. When the PLC scans the instruction is executed, the results so
11、on to be followed will be scanned into the instruction by the use, but also by CPU internal settings of the watchdog timer to monitor every scan exceeded the allotted time, to avoid the CPU internal fault makes a program run into the dead cycle. 3 .PLC implementation procedures and Features PLC impl
12、ementation of the program is divided into three stages, namely the input sampling stage, the program execution phase, the output refresh period. (1)Input sampling phase The input sampling stage, PLC to scan work order for all input input state is sampled and stored input image register, then enter t
13、he image register is refreshed.Then enter the procedure phase, the program implementation phase or other phases, even if the input state changes, enter the mapping register contents do not change, enter the state changes only the next scan cycle of the input processing stage to Pi sampled. (2)Progra
14、m implementation phase In the program implementation phase, PLC on the program in order to scan implementation. If the program uses ladder to represent, will always press on first post, the first left and right order. When the encounter program jump, then under the jump conditions are met to decide
15、whether the procedures jump. When the instructions relating to the input, output, state, PLC from the input image register and component mapping register read, the user process operations, operation results and then into element mapping register. The component mapping register, its contents will be
16、with the program execution process of change. (3)Output refresh period When all programs finished, enter the output of processing stage. In this phase, PLC output image register and the output of the state (output relay state) dump the output latch and through a certain way of output, and drive an e
17、xternal load. Thus, PLC in a scan cycle, the input state of the sample only in the input sampling stage. When the PLC into the program execution phase input will be blocked until the next scan cycle of the input sampling phase Caidui input state re-sampling. This approach is called concentrated samp
18、le, that is, a scan cycle, a period of time on the input state is sampled. In the user program if the output number assigned, the last time effectively. In a scan cycle, only the output refresh stage, the output state from the output image register output, the output interface refresh. In other stag
19、es the output state has been preserved in the output image register. This is called concentrated output. For small PLC, the I / O points less, the user program is shorter, generally concentrated sample, concentrate the output of work, although to some extent reduce the system response speed, but makes PLC work most of the time with the outside input / output device isolation, fundamentally enhanceHigh anti-jamming capabilities, increased
