[sdiy] Detecting the presence of a receiving MIDI device through current sensing

[Ben Bradley]

A shunt regulator like LM431 would do this automatically. It's a good thing
to use in a lot of places.


On Mon, Jan 27, 2020, 11:26 PM MTG <grant at musictechnologiesgroup.com> wrote:

> I like the last bit, the sensing. Certainly there will be current flow
> and voltage drop if anyone is attached.  The bit at the beginning about
> "constant" current flow ... seems to me you'd be adding edges from the
> circuit that is active while the MIDI one is not, so that introduces
> it's own issues. But it's all quite clever. I like it.
>
> I saw a Yamaha memory expansion that came with it's own battery-backed
> SRAM (separate from the synth's native battery-backed SRAM). Having seen
> inside a few PIN-pads and their tendency to "self-destruct" upon
> tampering I was thinking this expansion utilized executable code in the
> added battery-backed SRAM. Devious! It didn't turn out to be that, but
> one's paranoia gets the mind thinking...
>
> GB
>
> On 1/27/2020 8:07 PM, Didrik Madheden wrote:
> > I had an idle thought about reducing power rail interference in audio
> > circuits by equalizing the currents that any given part of the circuit
> > draws at any given time. For example, a MIDI transmitter may use up to
> > 5 mA when transmitting. If the circuit was also set to dump 5 mA into
> > a dummy resistor whenever it was NOT transmitting, that part of the
> > circuit would always draw exactly 5 mA, and no interference would be
> > injected into the power lines due to changes in load. Then I realized
> > that different MIDI implementations might draw slightly different
> > amounts of current depending on which optocoupler it's using, not to
> > mention what would happen if no MIDI device was connected, in which
> > case the circuit itself would inject the exact same amount of analog
> > noise it was supposed to prevent...
> >
> > This is an absolutely silly consideration. At least in this context.
> > In secure digital devices, power analysis is actually a field of
> > security research, where digital devices have secrets that are never
> > supposed to leave the chip, but which can be recovered by measuring
> > fluctuations in current consumption due to different parts of the CPU
> > causing different amounts of power use. I've also once made a Gameboy
> > program that plays audio through power line interference by purposely
> > turning on and off the CPU's power save mode in well timed sequence.
> >
> > But I digress. Normally, this is where my train of thought would end,
> > and I would go think about something else. This time, however I
> > remembered the discussions on this list about MIDI running status woes
> > where a device may get unconnected and reconnected. So I thought, hmm,
> > could the idea discussed above actually be useful?
> >
> > Consider this: 5 pin MIDI is a 1-5 mA current loop that typically goes
> > through a 1.6 V optocoupler and three 220 ohm resistors. So, by
> > sensing the voltage after the 220 ohm resistor on the +5 V line, you
> > could deduce whether a device is present on the other side. If nothing
> > is connected, you're just seeing +5V through a 220 ohm resistor, which
> > should be close to rock solid. If it's transmitting, by Ohm's law
> > there will be at least a V=I*R=0.001*220=0.22 V voltage drop while
> > transmitting a byte. With a bit of margin, you could detect anything
> > below 4.9 V as the presence of another device. You could for example
> > detect this by sampling the voltage with an ADC in a suitable moment.
> > Or, you could use a comparator with one input set to a 0.98*Vcc
> > voltage divider.
> >
> > A good time to check this would be when sending active sense commands,
> > which are meant for exactly this thing, just not exactly like this.
> >
> > This approach would have several downsides. It can't detect if a
> > device has been powered off but is still connected. It can't detect
> > anything about thru-connected devices. It would probably fail to
> > produce any useful information on non-powered MIDI splitter devices.
> >
> > What it *could* do, is to provide a basic heuristic for when a
> > transmitting device might suitably need to refresh the status byte, as
> > well as potentially resend CC's. Something that would nominally not
> > hurt anyway.
> >
> > It's silly. It's crazy. But has something like this ever been done to
> > anyone's knowledge?
> >
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