Can someone explain this circuit for me?

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Jay200MPH
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Can someone explain this circuit for me?

Post by Jay200MPH » Sun Sep 16, 2012 6:34 am

Sorry if this is kind of basic, but I can't figure this out:

Image
(Schematic from here.)

Why are there two inverters in immediate succession on the MIDI out line? Doesn't that just reset the signal to its original value? Would there be any consequences (aside from being 'not in spec') if I left them out and just connected the pin directly to R3?

I only want to make a quick & dirty MIDI OUT cable, I'm not concerned with the IN side of the circuit.

Jay

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Re: Can someone explain this circuit for me?

Post by Don T » Sun Sep 16, 2012 7:07 am

Not 100% sure, but I would imagine it's there for impedance-matching purposes...
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Re: Can someone explain this circuit for me?

Post by Stab Frenzy » Sun Sep 16, 2012 7:28 am

Agreed, I imagine it's used to buffer the output so that the midi port doesn't need to rely on computer manufacturers providing a game port that can push enough current down the midi cable.

You could try leaving it out and see how it works, it might be fine without it on your system.

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Re: Can someone explain this circuit for me?

Post by Jay200MPH » Sun Sep 16, 2012 7:43 am

Cool, thanks guys.

I suppose the resistor is there to prevent over-voltage spikes? I.e. it would be a bad idea to decrease it or leave it off?

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Re: Can someone explain this circuit for me?

Post by Stab Frenzy » Sun Sep 16, 2012 8:52 am

They're current limiting resistors not voltage, you want to leave them in. They're only 220 Ohms, why would you want to decrease their value?

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Jay200MPH
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Re: Can someone explain this circuit for me?

Post by Jay200MPH » Mon Sep 17, 2012 8:38 am

Thanks for the help, I decided to build it the "right" way with a 7406N logic chip I harvested. See here.

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Re: Can someone explain this circuit for me?

Post by schmidtc » Mon Sep 17, 2012 6:01 pm

The 2 not gates just act as a buffer, and could just as easily be an op-amp follower. Also, the opto-isolator is there to prevent ground loops.

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Re: Can someone explain this circuit for me?

Post by rschnier » Mon Sep 17, 2012 10:50 pm

Pin 5 on the MIDI out port needs to be able to sink (i.e., draw in) about 6 milliamps of current in order to light the LED in the other synth's MIDI in port. Some computer data interfaces (pin 12 of the UART jack) can't sink that much current, so a current amplifier (buffer) is placed in-between just to make sure . There are non-inverting buffers made, but this circuit uses two inverting buffers in series to achieve the same result. Often this is done because the buffers/inverters come 6 to a chip and there are often "unused" ones available from a chip that's already present on the board. Or because the inverted buffer chips are cheaper / more available. Or because the designer just felt like it.

Edit: And you definitely want to leave the 220 ohm resistors in there. The LED in the optoisolator likes about 5-6 mA in order to light up, and by having the three resistors in there, the current is approximately [1000 * (5 - 1) / 660] mA. (The "1" being the voltage drop across the LED in the driven unit's optoisolator.)
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Re: Can someone explain this circuit for me?

Post by Jay200MPH » Thu Sep 20, 2012 5:02 am

^^ Thanks for the great post! That makes a lot more clear what each component's job is. I couldn't find much info like that at all when looking around, just pages that said "build it this way because that's how the design is." I hate that.

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