Tach / rev counter input voltage

Just an idea - is the input signal to the tach / rev counter simply a 12v square wave?

The reason I'm asking is because at present my tach is not attached, which is quite annoying! The Haltech E6K engine management system provides a 5v square wave from its tacho output (as far as I am aware), so I'm thinking it may just be possible to boost this signal and use the standard tacho?

I have a 1971 240z btw, HS30-00829

If not it looks like having to go the autometer route...