Article, understand the difference between the single-chip microcomputer and PLC program design -

by:Coolmay     2020-07-04
Single chip microcomputer and PLC program design has the very big difference, MCU are all application users design of PLC program including system ( Siemens is referred to as the operating system) And the user program. Operating system used to organize has nothing to do with the specific control task of all of the function of the CPU. Tasks including handling of the operating system startup process, refresh image input/output process, calls the user program, interruption of detection and block of call interrupt error detection and processing, management, storage and communication tasks. PLC of many functions, such as scanning way, timer function and interrupt handling, is that the operating system. When drawing flow, should consider the problem. Switch quantity control system of PLC sequence function diagram is used to describe more concise. Use flow charts to describe the user program of PLC controller is also possible in principle, but should distinguish between function is done by operating system, which is the user program.

, for example, was asked whether need to wait for a signal with jump instruction for rising along the emergence of a microcontroller programming thinking. PLC scanning way at the beginning of each scan cycle to read all of the switch quantity input, according to the state of the two scan period before and after the same input point changes to determine whether there is rising. So there's no need to use back jump jump instruction to wait for the emergence of an input signal. If using this method to query the signal of the change of state, PLC can't do anything else, because it's all the energy used in query! And this kind of query was conducted in a scan cycle, will make scanning cycle is very long, it is easy to cause monitoring timer ( Watchdog) Timeout error. In the user program of PLC, only need to use a testing rising along the instruction can capture the pulse signal along the rise. If required quick response, can use the interrupt way, in the rise of the pulse signal along by system program calls the user to write interrupt program.

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