Dual input to handle hooking up to magneto p-leads where one lead may be grounded during run up testing.
Dynon's can handle 12 volt directly. For a magneto p-lead they use a 30K resistor. Jabiru connect via a fuse to an alternator coil. Rotax to the 5th trigger coil via a 30K.
Magneto p-lead can be +/- 200 volts. Need series resistor, maybe a voltage dividing potentiometer and probably external clamping diodes and a second series resistor. A pull down resistor needs to be in the circuit as well.
Want an RC filter that allows at least 500 hertz to pass through to suppress ringing being counted. See example of ringing (~8KHz) and high voltage (yellow trace) here.
Dynon SV EMS -
The SV-EMS-220/221’s Standard Voltage RPM inputs can read frequency-based RPM signals, provided the peak voltages goes at least 5.1 volts above ground, and crosses back down below 2.0V relative to ground. If the peak voltage exceeds 50 volts, use the included 30 kΩ resistors as described in the P-lead pickoff (Lycoming and Continental) Section above.
For signals that have a peak voltage of 12V or lower —such as Light Speed ignition outputs— use the low voltage RPM inputs. These inputs require that the peak voltage goes at least 2.1 volts above ground, and crosses back down below 0.8V relative to ground to be counted as a pulse.
Tach input low and high voltage details from dynon forum:
The low voltage input has 25K of impedance to ground when the voltage is between 0V and 4V. Above 4V, it has 1K ohm of impedance. This is why you're seeing it limit where it is. We never designed the low voltage input to be a high impedance load to the signal, just one that could handle a signal in the 0-12V range with an assumption the source impedance wasn't significant. The loads on this input are there for a variety of reasons including ESD protection and large overvoltage survival.
You should only need the low voltage inputs if your signal never goes above 10V. If it goes above 10V, the high voltage will work.
The high voltage inputs are 60K to ground up to about 9V, and then 35K to 3.3V. This appears it will work fine with the P-Mags and EI commander in parallel unless the EIC loads significantly.
Your final option would just be to put a resistor in series to the low V input. We only need a peak of 2V on this input to trigger.
And with regards to varying resistance to get 'clean' tach signal, dynon says this:
... reduce amplitudes with a straight resistor divider.
A P-lead signal has a big spike for the main pulse, then a trail of rings after that. As RPM increases, the amplitude of all of this increases. Eventually, the rings get big enough that they count for real signals and get counted.
Upping the resistance reduces all signals and reduces the amplitude of the ring below our threshold.
The external resistor keeps the voltage and current low in the supply wire limiting RFI. Also it limits the danger of accidentally grounding the ignition and losing power.
May make sense to isolate ground return on divider, rather than use ground plane.
Dual input to handle hooking up to magneto p-leads where one lead may be grounded during run up testing.
Dynon's can handle 12 volt directly. For a magneto p-lead they use a 30K resistor. Jabiru connect via a fuse to an alternator coil. Rotax to the 5th trigger coil via a 30K.
Magneto p-lead can be +/- 200 volts. Need series resistor, maybe a voltage dividing potentiometer and probably external clamping diodes and a second series resistor. A pull down resistor needs to be in the circuit as well.
Want an RC filter that allows at least 500 hertz to pass through to suppress ringing being counted. See example of ringing (~8KHz) and high voltage (yellow trace) here.
Dynon SV EMS -
Tach input low and high voltage details from dynon forum:
And with regards to varying resistance to get 'clean' tach signal, dynon says this:
The external resistor keeps the voltage and current low in the supply wire limiting RFI. Also it limits the danger of accidentally grounding the ignition and losing power.
May make sense to isolate ground return on divider, rather than use ground plane.