The Basic Distributor trigger is a very simplistic decoder that expects input like what a traditional distributor outputs. That is, 1 pulse per cylinder per cycle.
For this reason the signal lacks any cylinder position signal and so 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, provided it is conditioned appropriately to 0v-5v. 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.