[sdiy] MIDI input resistor

[Harry Bissell]

Yes, the reduced drive is only on the PIC side.

If you were worried you could use a cmos buffer and have the pullup resistor value just about anything you like...

If it works with the PIC you are golden

H^) harry

----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Harry Bissell <harrybissell at wowway.com>
Cc: Ullrich Peter <Peter.Ullrich at kapsch.net>, synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 20 Mar 2012 10:12:57 -0400 (EDT)
Subject: Re: [sdiy] MIDI input resistor

Ok, this is sounding like I should have the MIDI input circuit with a couple of 5K pots in it and a scope on the output and keep twiddling until I can see exactly what's going on.

Just to be clear, the 'reduced drive' would only be an issue for the rest of my circuit that follows this MIDI input (in this case a 16F688 PIC's UART). As long as that can get the signal clearly, there's no problem, right? Or are you talking about gain on the MIDI signal coming in, in which case more gain would enable me to receive weaker signals better?

Thanks,
Tom



On 20 Mar 2012, at 12:41, Harry Bissell wrote:

> sort of..
> 
> the resistor at pin 7 does decrease the turn off time, at the expense of lower current gain in the phototransistor. This
> reduces the amount of collector current (in saturation) so you need to make the pull-up resistor larger as a side effect
> (less current output is available.
> 
> The trade off is decided by the designer according to their needs... more speed or more drive.
> 
> Optocouplers have a pretty large variation in CTR (current transfer ratio), so much so that they are often 'binned' by this parameter
> and you can buy higher gain devices (usually at higher prices). So from manufacturing lot to lot, you might tweak those values...
> 
> H^) harry
> 
> 
> ----- Original Message -----
> From: Ullrich Peter <Peter.Ullrich at kapsch.net>
> To: Tom Wiltshire <tom at electricdruid.net>, synthdiy diy <synth-diy at dropmix.xs4all.nl>
> Sent: Tue, 20 Mar 2012 07:36:22 -0400 (EDT)
> Subject: Re: [sdiy] MIDI input resistor
> 
> Hi Tom!
> 
> The purpose of this resistor is to free the base pin of the output transistor from electrons when switching off with the 
> effects that it switches much faster. I first saw this resistor in the maplin midi thru box schematics and made some tests - and it 
> works perfectly with these resistors added! 
> 
> I built some midi interfaces and midi patchbays over the years and had problems without this resistor on some units. Especially if you 
> build a midi chain of lets say 3 or 4 devices as the midi serial pulses get more and more distorted due to slow switching optocouplers.
> It is normally no problem if you don't build a chain, but I also add it here.
> 
> A midi patchbay/matrix of course needs this resistor for fast switching as adding a patchbay is already a midi chain with 2 units.
> 
> And it goes somehow hand in hand with the resistor on Pin6. With the resistor on Pin7 added you can have higher ohm values on the 
> output resistor on pin6 (and saving current and maybe reducing negative EMC effects).
> 
> Ciao
> Peter
> 
> http://www.ullrich.at.tt
> http://go.to/datadial (Synth addon)
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Tom Wiltshire
> Gesendet: Dienstag, 20. März 2012 12:26
> An: synthdiy diy
> Betreff: [sdiy] MIDI input resistor
> 
> What's the purpose of the resistor from Pin 7 to ground seen in some 2N138/139 MIDI In optoisolator circuits?
> 
> Some schematics have it:
> 
> 	http://www.ucapps.de/midimon/midimon.pdf
> 
> And others don't:
> 
> 	http://www.sparkfun.com/datasheets/DevTools/Arduino/MIDI_Shield-v13.pdf
> 
> We've discussed the various values of the resistor between the output pin 6 and Vcc here before, so I won't go back over that. The summary IIRC was that different manufacturer's versions of the 2N138/139 seem to require different values, so pick something that works between 270R and about 1K8.
> 
> Thanks,
> Tom
> 
> 
> 
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Harry Bissell & Nora Abdullah 4eva