That is an excellent description of how the tach works.
I made a mistake in the earlier msg the correct way is that pin 1 gets the signal from the coil and pin 15/54 connect to ign.
The reason I am asking is that I have a newer engine in my car which has digital tach output, I am trying to find a way to make the original tach work with the newer engine. ( I know many will hate me for swaping my 3.5 engine, trust me I hated to do that but I had no choice, please forgive me).