HollySys PLC - the usage and principle of PLC programming interrupts

When the PLC understands the interrupt function, it needs to understand the PLC's operating cycle or scan cycle. It is necessary to talk about the process of PLC sequential control loop execution, which is a prerequisite for understanding interruptions and must be mastered. It is divided into three parts, input processing, program processing, and output processing.

1. Input processing: Before the program is executed, the programmable controller will read the ON/OFF status of all input terminals of the programmable controller into the input image area, even if the input changes during the program execution, the content of the input image area will not change, and the changes will be read when the input processing of the next cycle is executed.

2. Program processing: PLC reads the on/off state of each software component from the image area of the software component such as the input image area according to the instruction content in the program memory, and then operates sequentially from step 0, and writes the result obtained each time to the image area, therefore, the image area of each software component gradually changes its content with the execution of the program. In addition, the internal contacts of the output relay perform actions based on the contents of the output image area

3. After the output processing and all instructions are executed, the on/off state in the output Y image area will be transferred to the output latch memory, which will be used as the actual output of the programmable controller.

The time it takes to perform an action is the operation cycle, also known as the scanning time. So what's the connection between outages and scan cycles? Interrupts are not fully executed in order from top to bottom, but rather that interrupt programs preferentially run programs individually, output immediately, and do not participate in the entire loop operation.

What is the purpose of the outage? We know that the PLC scan cycle is very short, so it is difficult for us to see the execution of the PLC sequence control. Here, we might as well assume that the scan cycle is 10s, i.e. it takes 10s to execute all the procedures. There is a very simple program LD X0, OUT Y0. When X0 is ON, Y0 will not output immediately, but will output after 10s. When X0 is disconnected, Y0 does not shut down immediately. So in our practical application, the PLC can't work because it doesn't use any equipment. In this case, interrupt processing is required, and the input interrupt function is used to execute the input immediately. In fact, the running time of PLC is very short, only a few milliseconds, which can fully meet the needs. However, certain timings, such as high-frequency pulse input and pulse capture in the microsecond range, must be affected by periodic operation.

Therefore, if you want to complete the on/off state processing multiple times in a cycle, you must use the interrupt function. In general, we rarely use interrupts. Even if a PLC with interrupt function is required for high-speed instructions, there is no need to write a special interrupt subroutine. For example, a high-speed counter uses an interrupt mode to process high-speed pulses from a specific input relay, so it can count regardless of the operating time.

You can use the special comparison command of the high-speed counter to output the counting results immediately, or you can use the high-speed counter interrupt method to process the count value of the high-speed counter. High-speed counters are not used to count high-frequency pulses. We use the interrupt function to count high-speed inputs.

HollySys PLC - the usage and principle of PLC programming interrupts

HollySys PLC - the usage and principle of PLC programming interrupts

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