[sdiy] MIDI cable impedance and MIDI In ESD protection?

[Michael E Caloroso]

The MIDI spec specifies a maximum cable length of 50ft.  That is dependent
on the cable capacitance, which is not mentioned in the spec.

I own a ProCo 150ft snake cable that I used for live music, between the
stage and the FOH mixer.  During a recording session, I ran the snake cable
from the basement to the main floor.  There were MIDI devices on both
floors, and we wondered how well MIDI would work over that snake cable.  I
made adapters for both ends - MIDI worked flawlessly over 150ft of cable.
Later I found a spec for that snake cable and found that the cable
capacitance was very low.

MC

On Fri, Jun 28, 2024 at 12:04 PM Florian Anwander <fanwander at mnet-online.de>
wrote:

> Kay from LPZW did this already in 2017 in the Schleußig (
> https://leipzigwest.org/?page_id=173) ... ;-)
>
> Am 28.06.24 um 11:50 schrieb René Schmitz:
>
>
> If you have the space and budget, use two optocouplers: one for each
> direction, that allows you even to sense which direction it is.
>
> (So you can change the output too...)
>
>
> Where do I collect my prize? :)
>
>
> Best,
>
>  René
>
>
> Am 28.06.2024 um 00:06 schrieb Mattias Rickardsson:
>
> On Thu, 27 Jun 2024 at 20:55, Andrei Kudryavtsev <
> andrei.kudryavtsev at gmail.com> wrote:
>
>> Speaking of TRS mess, found this one in the recent application list for
>> MIDI innovations:
>> https://midi.org/innovation-award/psimidi
>>
>
> Oops, that has been solved in commercial products for over 5 years.
> But without those extra diode voltage drops!
>
> (Assuming 1.9 V drop for the optocoupler and 2 x 0.3 V drop for the
> Schottkys, and the new 3.3 V (+/- 5 % on the lower side) logic level,
> there's 0.635 V left for the 220 ohm resistor. The current is then only 2.9
> mA, far below the required 5 mA. Oops2!)
>
> (It would need to have 127 ohms instead of 220. But with 5 V (+/- 10 % on
> the higher side) logic level, and 1.9 + 0.3 + 0.3 V voltage drops, you get
> 3 V over the resistor. 23.6 mA is above the specified Abs Max input current
> of the 6N138 optocoupler suggested in the MIDI spec, even if the 6N137 used
> here will survive. Oops3!)
>
> I'm sure similar gender-agnostic TRS MIDI In jack solutions must have been
> implemented by several manufacturers by now? It was a bit weird to see
> different brands using different polarities when TRS MIDI emerged, and even
> more strange to see MMA omit a solution for it in the new standard. :-)
>
> Thanks for the reading tip, Andrei!
>
> /mr
>
> PS: I wrote "TS" in my first message, mistake - should have been "TRS".
> Sorry.
>
>>
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