@kb1gtt
Yes, I only suggested RS485 because RS232 was used with success. And because it was cheap and readily available. I really should verify the details first before making suggestions
I do have a bad habit of jumping to conclusion really quickly. But thanks for pointing the details out to me.
I agree with your points. But before I go further, I need to clear up what I think a VR signal should be. Is it true that the VR signal can go pass 5v at high RPM? Unless there is circuitry in the sensor itself, I don't see how voltage is limited in real world scenario since amplitude varies with RPM. The voltage is actually a function of the ECU inputs depending on how much energy is generated by the VR. If we don't have anything connected to the VR sensor, the voltage would ideally go to infinity. We only see a certain voltage with a scope because of the input impedance of the meter and whatever it is connected to, which is why it is varying with RPM or energy generated. It is current thats being generated. I understand the current limit of the Maxim IC since VR can only generate so much energy, so the current probably will never go beyond 40mA. And even though the Maxim chip says 5V 40mA limit on its inputs, I don't see anything in the functional diagram taking up to 40mA other than their clamping diodes once it go beyond 5V. So it seems we only need to limit the current of our ouputs since the Maxim VR chip is designed to clamp voltage to the rails up to 40mA.
But this is assuming an ECU uses the Maxim chip. We really don't know what stock ECU uses as their VR inputs.
For the Maxim IC, it seems we can still use the RS485 since its output is typically under +/-10mA. Also, if we want to be on the safe side and avoid going beyond 5V all together, we can just use LVDS drivers instead of going through all the trouble of generating a negative rail. Something like this:
http://www.mouser.com/ds/2/149/FIN1531-81456.pdf And we can apply current limiting resistor if needed. But if we choose a driver with under 40mA, which most are, we should be ok. Hmm, i'll give this more thought. But I think this is ok so far.
I think to model the VR sensor, we really need to figure out what is the maximum energy that is typically generated by the VR sensors in milliwatts. Then, we can apply limiting resistors to any differential output to match same maximum power output, no matter if it is RS485 or LDVS or logic outputs or OpAmp. So for the stimulator board, the VR output should have series resistors to make it possible to fine tune the power output, if needed. But I am confident that if we can get a ball park of what kind of power it typically generates, we will never need to change it. It doesn't seem like it needs to be that precise. If we limit the current, I think it is perfectly safe to apply it to any input that expects a VR sensor signal.
I further agree that VR signal doesn't have to be differential. It just needs to generate opposite current on the transitions. Differential driver just seems to match the VR model easily and cheaply, with only 1 component needed. Another method we can think about is to add a high pass filter with a transformer or common mode choke to the differential output. A differential output with a high pass filter will model the varying amplitude with frequency. Then driving that to the transformer models the current generation nature of the VR sensor. But maybe we are going way beyond what it's needed.
I like the simple differential driver more.
, but that doesn't matter. once you tested vr circuitry at the desired frequency range, for other purposes you could use simple 555 circuit to generate a saw on the vr-input pin of your discovery board…or use debugging feature in java console. 
That just doesn't sound right and should be illegal. Lol 
