Technical puzzle | how does PLC execute interrupt? How to apply it?

PART1

How to understand PLC Interrupt function of ？

When understanding interruptions , First of all, be clear PLC The operation cycle or scanning cycle , It's necessary to say PLC The process of sequential control cycle execution , This is the premise of understanding interruption , Must master , Divided into three parts ： Input processing 、 process 、 Output processing .

1、 Input processing

The programmable controller before executing the program , Connect all input terminals of the programmable controller ON/OFF The status is read into the input image area , Even if the input changes during program execution , The contents of the input image area will not change , The change is read when the input processing of the next cycle is performed .

2、 process

PLC According to the instructions in the program memory , The image of each soft element is read out from the input image area and the image area of other soft elements ON/OFF state , And then from 0 Start the operation step by step , And write the results obtained each time into the image area . therefore , The image area of each soft component changes its content gradually with the execution of the program , Besides , The internal contact of the output relay performs action according to the content of the output image area . 

3、 Output processing

After all instructions are executed , Output Y In the image area ,ON/OFF The status is transferred to the output latch memory , This is the actual output of the programmable controller . 

The time required to perform an action is the operation cycle, also known as the scanning time , So what is the relationship between the interrupt and the scan cycle , An interrupt is a complete execution that does not follow the order from top to bottom , Instead, the interrupt program gives priority to running the program processing alone, and it is output immediately, and does not participate in the operation of the whole cycle .

PART2

What is the function of interruption ？

We know PLC The scanning period is very short , So it's hard to see PLC Sequence control execution process , Here we might as well assume that the scanning period is 10s Time for , Is the need to execute all the procedures 10s Time for , There is a very simple program LD X0,OUT Y0, According to the above picture X0 by ON when ,Y0 There is no immediate output , But wait until 10s Output after , To break off X0 after , Again Y0 It's not closed right away , All need to be scanned before there are results . So in our practical application PLC There is nothing to use, and the equipment can't work , At this time, you need to interrupt processing , Adopt input interrupt function , Execute the input immediately . actually PLC The operation time is very short, only a few milliseconds , Can meet the needs completely , But some, such as high-frequency pulse input 、 The time of pulse capture at the microsecond level must be affected by periodic operation . 

So if you have to do it many times in a cycle ON/OFF State processing , The interrupt function must be used . In general, we rarely use interrupts , Even if needed PLC All high-speed instructions have interrupt function, so you don't need to write interrupt subroutine , For example, the high-speed counter processes the high-speed pulse from the specific input relay in the way of interruption , Therefore, no matter how long the operation time is, it can be counted , The result of counting or using the special comparison instruction of high-speed counter is output immediately , Or the count value of the high-speed counter is interrupted by the high-speed counter to execute specific program processing . 

High frequency pulses are counted without using a high-speed counter , For example, we use the interrupt function to count the high-speed input , Examples are as follows , from EI To FEND Is the main program , From the pointer I101( Input X0 Interrupt pointer for ) Start to IRET It's an interrupt subroutine , Whenever you feel X1 The rising edge of executes a subroutine to immediately transfer the value to D10 in , This avoids PLC The influence of operation cycle . 

In this paper, the source ：PLC enthusiasts

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