Receive MIDI in Arduino

Other Hardware General Electronics
1

Hi. I am developing a project which receives MIDI messages on Arduino UNO from RX pin (Pin 0).
To adapt MIDI signal to be received by Arduino I use an optocoupler, in this case 6N138. I have searched (in this forum and Google) how configure the adaptation circuit. I've found differents ways:

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1600×809 196 KB

Does anyone know explain why differences in resistance values​? Why a 1k resistor between pin 7 and pin 5 in the first configuration?

2

In the first circuit, when there is no current flowing in the input loop, the first transistor is off, so the 1k pulldown is needed to keep the second transistor reliably turned off, allowing the 470 pullup to define the output level (high). Perhaps it's not really needed in practice, due to the internals of the device, so the other circuits don't have it.

The 270 vs 470 pullup difference is not critical.

Using 470 at the input is unusual (in my admittedly limited experience). I'm pretty sure the official spec says 220.

3

Thanks so much.
Therefore, I could use this circuit:

Regards and thanks!!!