[sdiy] USB/MIDI ground isolation?

[Tom Farrand]

I don't know how that is addressed for a USB/MIDI interface but here
is a reasonable guess which is based on doing such things for medical
devices that must be isolated from a patient.

A USB interface can supply 100mA through a non-powered hub or 500mA
with a direct connection (or via powered hub).  The +5V on a USB port
can be used to create an isolated voltage power supply with
"significant" isolation.  A DC-DC converter is used for the onboard
power supply and a toroidal transformer provides the isolation.  The
secondary of the transformer has a full-wave rectifier and filter
capacitor.  That can be run into low-current regulator for powering
the interface components for the MIDI signalling on the isolated side.

But what about getting the data across the isolation barrier?  There
are several solutions available.  One solution uses a standard
optoisolator.  Another solution would use optically coupled FETs to
provide on/off control.  Still another solution would use Analog
Devices' isolation chips.  These accept digital signals on the inputs
and provide digital (isolated) outputs.  They work on the principle of
turning a data stream into RF bursts at about 300MHz.  The signal only
has to be sent/received on chip ... a very short distance.  I used
such devices on a commercial medical product to great success.

This solution is maybe a bit over the top for a grunt USB/MIDI
interface but would provide nearly perfect isolation.  Ground and data
for the MIDI side would have no connection with the USB host
whatsoever.

Now I am curious.  I'll stay tuned and see if anyone else has cracked
open such an interface to see if what I suggest has been done.  If not
... thanks for a nifty product idea!  (The device to which I refer was
only a bit larger than your average thumbdrive.)

Peace.
Tom Farrand


On Wed, Sep 16, 2009 at 5:18 PM, <johnspeth at yahoo.com> wrote:
>
> We all know that the 5 pin DIN version of MIDI provides isolation between grounds by virtue of the optocoupler.  Can anyone tell me how they addressed the ground isolation problem with USB/MIDI?
>
> Thanks, John Speth.
>
>
>
>
>
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