This is a signal that lacks the cylinder position signal. Without a missing/added tooth or camshaft signal reference, Speeduino cannot calculate crankshaft angle, phase of cycle, or cylinder assignment. A distributor must be used to route the resulting sparks to the correct cylinders. The signal can be as simple as the breaker points from an old pre-electronic distributor, to a crankshaft wheel without any abnormal, extra, or missing slots. Most who have installed aftermarket tachometers are familiar with the simplicity of the signal with the only variation being the number of pulses in each crankshaft rotation.