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Thread: MIDI DIN in: Why is the PC900 optocoupler not recommend for Teensy 3.x anymore?

  1. #1
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    MIDI DIN in: Why is the PC900 optocoupler not recommend for Teensy 3.x anymore?

    If you take a look at the MIDI DIN in schematics for Teensy devices, you'll see that a different optocoupler is recommended for Teensy 2 and Teensy 3.x.

    About three years, that same page recommended the PC900 for both T2 and T3. Does anyone know why this has changed? Is there an issue running the PC900 (H11L1 in my case) at 3.3 volts?

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by yeahtuna View Post
    Does anyone know why this has changed?
    Sharp discontinued the PC900. Many people asked me to update the page to a part which they could actually buy.

    At that time, 6N138 seemed to be the most readily available part, and most likely to remain available long-term. I also happened to have a small tube of them in my parts bin.

    In hindsight, maybe just replacing PC900 with H11L1 would have been simpler?

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    I think it's fine that you're specifying the 6N138. My motive for asking here is that I have customer experiencing some issues with dropped MIDI messages through my MIDI in DIN and I'm looking for the culprit. If I post my schematic, do you think you could take a look and give me some feedback?

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    Here's the schematic. The only thing I'm entirely sure about is the resistor on Vo.
    Click image for larger version. 

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    Opps. I missed 'not'. I meant to say the "The only thing I'm not entirely...

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    Senior Member PaulStoffregen's Avatar
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    I have never used H11L1, so I can't speak from experience.

    I looked quickly at the On Semiconductor datasheet for this part. It specifically mentions 270 ohms in the Vol spec.

    Click image for larger version. 

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    One thing I did notice is the other similar parts have higher LED current thresholds. Is there any chance you accidentally used the "L2" or "L3" version of this chip? That would be a very plausible explanation for it sometimes not working.

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    Thanks for looking into this Paul. I'm guessing the issue much be related to that particular customer's system.

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