Counter scanning can be used to scan each saved input in turn, starting with the calling source (highest priority) with 1. When encountering a high level, that is, a call with a calling source, the corresponding calling source encodes and decodes and sends it to the output port to display its corresponding calling number; After a certain delay, scan the next call source. If there is no call, skip the call source and scan the next call source. Therefore, the display between all call sources will not be too long, but there can be a quick corresponding display. In this way, the calling source 1 to the calling source 5 is continuously scanned circularly, and the scanning times are controlled by the counter. That is, after all five call sources are scanned once, the counter is cleared and the next scanning is performed. Display the calling number continuously and circularly.
Another method is to send every input signal that has been input, processed and saved to its next AND gate, and then send it to the priority encoder when the strobe signal arrives. Input signals from I 0 to I 9 are encoded into 10 corresponding BCD codes, with I 9 having the highest priority and I 0 having the lowest priority, which effectively prevents other channels from interfering with the input. The generation of strobe signal consists of five RS triggers. In fact, the strobe signal is a sequential pulse, and the ring counter composed of shift registers has only one cycle state in each state. The scheme uses self-feedback logic circuit for self-starting, which has the advantages of no need to add decoding circuit and relatively simple structure.