[Lorenfb]

Actually it's very simple:

1. Low impedence tachs are driven from the points and some require a 33 ohm
pullup resistor in the CDI versus the 100/120 ohm resistor.
Note: These will result in a bouncing tach if connected to a CDI output.
Also, a low impedence tach used with a 100/120 ohm type of CDI may result
in the CDI not working requiring an additional pullup resistor.
2. Some early tachs (high impedence inputs) are driven from the CDI output,
i.e. the coil wire.
3. Most all late, e.g. 911SC 6 pin CDI, tachs are driven from a signal output of the
ignition unit or the engine's ECM unit.
Note: These will result in a bouncing tach if connected to a CDI output.