tach misreading - showing wrong rpm's

Before you go to the trouble of adding a tach... How does the MSD system work? Can you plug in a laptop or other devise to see timing and ign setting real time while the engine runs? I have a speedhut tach and it is always accurate with my FAST xfi/com RPM signal. Also, does tha MSD system have a rev limiter set? I would think even if the tach was off you'd have hit the limiter if you had one.

Joe