1993 Miata Tachometer signal for a 1.8 Turbo swap

I am in the process of infusing some much needed horsepower into everyone’s favorite Japanese roadster. I am installing 96 1.8L motor into my 93 chassis, installing a Flyin Miata no electronics turbo kit and using a Megasquirt MS3x ECU built by MSLabs(reverant) on the MiataTurbo forums. I want to keep the 1.8 ignition coils so I don't have to swap the 1.6 coils and fab a bracket. I have figured out what I need to wire up to power the coils. However the one issue is the 93 coils/igniters provide a tach signal to 93 dash tachometer. The 96 coils do not do this, the 96 tach receives its signal from the ECU. The Megasquirt can output a square wave signal (20Hz at 1000rpm).

The big question is what is the 93 Miata Tach expecting for a signal? Could I simply connect the MS3x output to the 93 tach and be in business?

I have searched all over the net for any info I can find on the OEM tach with little luck. Before I start probing wires with a borrowed oscilloscope I figured I would ask the experts on here.

Build thread located at the following thread:

http://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1993-mazda-miata-18-swap-flyin-miata-turbo-kit/88733/page1/

I've been playing with NB gauges for a side project, and from what I've found, the tach expects a wire to be pulled to ground. I got it working reliably in conjunction with a pullup resistor. It's nice because I can use 5v logic (Arduino).

Here's some good info: https://www.flyinmiata.com/support/instructions/misc/revlight.pdf