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6n138 Before and After | by Wooster Audio
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6n138 Before and After

For my MIDI projects I've use been using 6n138 optocouplers. So far they have all been 5 Volt designs, and the standard schematic with a 270 ohm pullup from the output (pin 6) to Vcc works fine.

 

However my FPGA synth is 3.3V, and I noticed that I've been losing MIDI data and getting UART framing errors. I looked at in the scope and YIKES it looks terrible (top picture) .

 

I saw this link:

 

blog.gg8.se/wordpress/2009/07/26/fixing-gameboy-powered-a...

 

and tried a 4.7K from VB (pin 7) to ground and it looks so much better (bottom picture).

 

For this test I modified one of my MIDI-IR's to be a signal generator.

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Taken on August 24, 2010