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I was just saying they weren't low impedance wires to start with. Probably won't be able to mess with it until Sunday now.
Wow, actual experimental evidence, and not just conjecture (however well based) - nicely done....so I set up a spare 4n25 on a bread board...
Wow, actual experimental evidence, and not just conjecture (however well based) - nicely done.
For good app notes, try a google of "optocoupler frequency response". AN3009 at this link explains the rise/fall switching behavior.
If you still have your breadboard setup, I'm curious how much filtering the 0.01uF cap (C11) on the opto output provides. My back-of-the-envelope suggests that it's starting to roll off the signal at 3.5kHz or 3500rpm with a 60-tooth wheel (it should be fine with a 4 vane disti).
Your comments got me thinking more about the opto isolator and the fact that the response curves in the application guide and their testing conditions don't match up with the way that the isolator is applied in MS2, so I set up a spare 4n25 on a bread board powered up at 5 volts with the same resistor sizes as used on MS2. I triggered it by ground switching the diode through an open collector transistor (the way I have it set up in MS2) driven by a wave form generator with a 100% offset square wave.
When the output of the 4n25 transitions from low to high (the opto output transistor turning off), the output has about a 35 micro second delay based upon the way the application guide defines the transition time (90% of final value). This was consistent from a test frequency of 1 kHz to 8 kHz (looking pretty ugly by then). So, my measured transition delay is in that 25 - 50 micro second ball park that I had assumed from the curves in the application guide. However, what was really interesting is that when the output transitions from the high state to the low state (the transistor turning on), there is no perceptible transition delay (up to my test frequency of 8 kHz). That kind of buzzes my brain a bit because the application guide indicates that the output delays on both transitions are similar in duration (but, not identical). Also, I would have expected more of a delay in turning the transistor on rather than shutting it off.
Based upon my very limited experiment, if your 1st tooth and tach signals are based upon the falling edge transition, the opto isolator is not going to introduce any material delay up to my test value 8 kHz which would easily handle a 62 tooth crank wheel at 7000 RPM. However, try a 62 tooth wheel at 7000 RPM with the rising edge transition and you would probably be spam in a can. By dumb luck rather than trying to avoid the delay in the opto isolator my tach signals are based on the falling edge transition which may explain why they are rock steady.
I'm pretty sure it's a mid-stage low-pass noise filter, probably setup for 4-vane distributors or low tooth count wheels. As you found, it's not suitable for a 60-2 tooth wheel.Do you know why C11 is in the circuit? It is not clear to me what function it is performing and it is clearly degrading the ability to get reliable edge detection off of a rising edge at high pulse counts.
For those with less Arduino talents, a JimStim set to 24/2 (Nippondenso) is close enough to simulate a Yoshifab DSM CAS with a hi-res disk (it works but there will be a LostSyncReason in the logs if you look) - see link... wrote a little bit of code on an Arduino Uno to emulate the Yoshifab 24/1 disc with the exact same phasing between the tach and cam windows that exists on my disc.
Yeah, based on your results and the circuit, the C11 filtering causes this and must contribute to the occasional posts of flakey DSM CAS behavior. Changing from 0.01uf to 0.001uf would leave some filtering in place but should greatly reduce the phase shift.at 12,000 RPM you could clearly see on the oscilloscope traces that the distorted leading edge of the cam tooth signal was starting to significantly overlap with the next trailing edge of the tach tooth
If you're talking about the fishtank demo, the "sealed" MicroSquirt was a fraud -- it was specially prepared for the demo to prevent water getting in. The off-the-shelf uS can be opened with just a philips. The circuits are all surface mount so it's difficult to modify.If you are a microsquirt user, particularly if you have the sealed version, this is of no value since you can't get access to modify the opto circuit.
The comments in that MSExtra thread aren't real accurate. The DSM CAS with 24/1 wheels (no missing tooth), is finicky about exact edge alignment. Per a different thread here, the local experts say that the cam wheel only needs very loose alignment when used with a missing tooth crank wheel (e.g. 60-2), but needs tight alignment with non-missing tooth wheels.The final comment about the opto in circuit in this MSExtra thread is interesting.
If you're talking about the fishtank demo, the "sealed" MicroSquirt was a fraud -- it was specially prepared for the demo to prevent water getting in. The off-the-shelf uS can be opened with just a philips. The circuits are all surface mount so it's difficult to modify.
There's nothing wrong or bad about using an optocoupler with a Hall sensor, as long as the filtering is appropriate for the RPM range and tooth counts.