[sdiy] Updated MIDI Spec for 3.3v

[Tim Ressel]

In every midi circuit I've seen there is a 220R resistor on the receive 
side and 2 220R resistors on the send side. If you run through the 
calculations that makes for 5mA LED current with a 5 volt transmitter. 
For a 3.3 volt transmitter reduce the send side resistors to 51R (49.9R) 
and you'll still get you 5mA LED current.

--timbo


On 8/11/2017 8:33 AM, MTG wrote:
> I found out from the guy at MIDI Solutions since he's making some 
> tweaks to his products.  There are also a few blog entries on the 
> MIDI.org site about a new standard smaller connector, and a few 
> comments about what people "out there" have been doing with 3.5mm 
> stereo jacks.  Not sure that's the way they'll go but it would be nice 
> if they picked a connector that's easily panel-ized, inexpensive, 
> available and isn't already used for something else in our repertoire.
>
> That doc I sent a link to was from a 3rd party site (and a .DOC as 
> well).  I didn't bother looking on MIDI.org for PDF.
>
> GB
>
> On 8/11/2017 7:52 AM, Neil Johnson wrote:
>> I remember discussing this very topic on the midi forum years 
>> ago....and it took them until last year to finally publish something.
>>
>> If I remember rightly it was on a thread about drawing power from MIDI.
>>
>> Neil
>>
>> On 11 Aug 2017 15:26, "MTG" <grant at musictechnologiesgroup.com 
>> <mailto:grant at musictechnologiesgroup.com>> wrote:
>>
>>     Maybe I'm the last to know, but there appears to be an official word
>>     on this:
>>
>>     https://mitxela.com/other/ca33.pdf 
>> <https://mitxela.com/other/ca33.pdf>
>>
>>     I know people have been doing their own thing regarding the series
>>     resistors for the current loop.  I also noticed this caveat here:
>>
>> https://www.midi.org/forum/492-certain-midi-equipment-for-3-3v
>> <https://www.midi.org/forum/492-certain-midi-equipment-for-3-3v>:
>>
>>     "The MIDI specification specifies a 5 mA current loop, and shows
>>     three 220 Ω resistors to limit the current when used with a 5 V
>>     transmitter.
>>
>>     When you have a 3.3 V transmitter, the two resistors in the
>>     transmitter must be smaller to get the same current, as shown in the
>>     Electrical Specification Update.
>>
>>     The receiver does not need to know what voltage the transmitter
>>     uses; it always uses the same 220 Ω resistor. All transmitters are
>>     designed to work with that same receiver circuit. This means that
>>     all transmitters and receivers are compatible.
>>
>>     (The receiver can use either 5 V or 3.3 V as supply for the
>>     optoisolator; this part of the circuit is isolated from the current
>>     loop, and does not affect compatibility of the MIDI interface. But
>>     note that only the PC900/H11L1 optoisolators can run at 3.3 V; other
>>     common models like the 6N137 or 6N138 require 5 V.)"
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com