You mean an ACH transaction processing system? Zenpayroll has a nice series of blog posts: http://engineering.gusto.com/how-ach-works-a-developer-persp...

http://engineering.gusto.com/how-ach-works-a-developer-persp...

http://engineering.gusto.com/how-ach-works-a-developer-persp...

http://engineering.gusto.com/how-ach-works-a-developer-persp...