Sequential (21) level trigger
https://wodewangzhishime.blogspot.com/2022/10/httpswodewangzhishime.html . . Ref: https://kamchihau.blogspot.com/2023/02/blog-post.html . . . Me: This article mention "level" triggering . https://wodewangzhishime.blogspot.com/2023/02/if-ipaddress133133-nathon-road.html . . . Serial input = edge trigger. . Left or right shift. Parallel input = level trigger. . No left or right shift. . The output is ouput to the output line.