Sam,
In following along I'm not sure I caught all what you said in that will the 115 4 stroke work with the same tach on the same settings as would a 2 stroke if one were to upgrade his engine to a newer model.....aka digital to analog conversions are internal to the engine........without having to change the tach?
Well, not sure exactly how to answer but.........
Will his original tach work? It should signal wise, but can it cal to a 12 pole stator? Not sure, he hasn't discloses if his original tach has any if at all pole (or cylinder) settings..........There is some over lap between poles and cylinder settings on tachs.
As far as digital and analog, read on if you dare.....
His 115's 4 stroke tach's source signal from the ECU is only "digital" in the sense of yes, it does assumingly have a processor managing/generating it BUT! As processed, the tach signal is just periodic logic level pulses (0V/5V or 0V/12V) that change in frequency dependent on the engines rev's....that's it.
With that said and as far as the testing I've performed with the "converter" and its function within the system, the ECU's tach signal and original tach are NOT "digital" per-se in any part of a/the more traditional sense of a digital serial stream of binary words with addressing, data packets and such.
The "converter", contrary to popular belief here and afar, it's NOT converting digital to analog(or vice versa)......far far from it!! It's just a inverter that by default is being fed, and might be also be level shifting 5V pulses as it uses a 12V supply, so by default, it's totem pole output rails are tied too and output approx 0V and 12V.
Pretty much any modern'ish off shelf tach (automotive 8/6/4 cylinder, marine 12/8/4 pole) would work hooked directly to his ECU's tach's output due to the ECU just outputs a simple frequency varying pulse train.
This is how all basic automotive and marine tach's we use and see work, call the signal analog or digital, they just simply require varied freq pulses be it from a coil, a processor, a plug wire, a lego® with a attached magnet spinning about creating pulsing in a near by hall effect sensor, proximity sensor near rotating gear teeth it doesn't matter, just cal (we're used to switches 8/6/4 cyl or 12/8/6 pole depending on our app) them to display what they should read based on the app, see math above....
The "converter" Merc installs, as discovered (the one I bought and tested anyway) is used as best I can tell to just perhaps level shift from 5V -> 12V.
If used, 5V signal is probably on the low side of some tach's required threshold so a/the converter bumping it to 12V doesn't hurt in that it assures more of a definitive signal level for a wider spectrum of tachs of which, also gives even higher hysteresis thresholds........ If in fact the ECU puts out only 5V logic, bumping(converting) up the signal level from 5V to 12V as close to the ECU as possible, also gives the signal more noise immunity in that any noise picked up in the wires remains constant but swinging from 0v to 12V as opposed to 0V to 5V as it runs to it's destination (the tach) improves the signal to noise ratio (SNR).
Also, this "converter" inverts but, who cares? the tach doesn't. The converter is probably also being used as a current buffer (line driver), due do running wires long distances not only picks up more noise, it adds additional capacitive loading and allowing for higher sinking and sourcing currents improves signal integrity.
