> From: eric@...
>
> Well I've figured out the "clicking" thing... and the Polaris
> isn't broken!
>
> AFAIK the MIDI spec calls for the input on a device to be
> nothing but an optoisolator conected to pins 4 and 5 of the
> DIN socket... no other connections. But some newer/cheaper
> MIDI interfaces, including the one I was using, "cheat" and
> steal power from the MIDI input by grabbing +5 volts off pin
> 4 and using the shell as negative.
>
> Well in the Polaris, the MIDI output connector shell is
> connected to the chassis, and only to the analog/digital
> grounds through a 1-megohm resistor. So when current flows
> through the shell, it energizes the chassis relative to the
> digital + analog grounds. The current flow varies depending
> on whether data is or is not being sent out the MIDI port and
> I think that was causing the clicking I heard. Just for
> reference, with nothing plugged into the MIDI port I measured
> a few hundredths of a volt between chassis and analog ground
> (at the PS board), with the interface plugged into the port I
> measured almost a volt!
>
> It seems like what needs to happen is (other than
> manufacturers making MIDI interfaces that follow the spec,
> and good luck with that) to connect the MIDI connector shell
> to digital ground. But doing that with the Polaris in the
> factory configuration would result in digital ground being
> connected to the chassis which I think would be bad for a
> number of reasons. The only solution to ∗that∗ problem I can
> think of is to replace the MIDI output connector with an
> isolated one and I'm too lazy to do that. ;^)

Interesting. The shell is connected to the chassis, but nothing in the cable is supposed to be connected to the shell. The shield is supposed to be connected to pin 2 (the middle pin) at each end, which is connected to digital ground in the transmitter but left disconnected in the receiver. If you measure 0 ohms between the shells of both plugs, then it's the cable that's violating the spec, and tying the two chassis grounds together, which defeats the purpose of the optoisolator.

A small completely floating MIDI device could safely ground itself to pin 2 of its input. Taking power from pin 4 doesn't necessarily work, though, because the spec doesn't actually require that pin 4 be connected to +5V and pin 5 be driven from an active low output through resistors; that's just a suggestion. Pin 4 could be connected to an active high output and pin 5 to ground through resistors.

But if you've got something that's taking MIDI in and converting it to USB, then it ought to be powering itself off USB, in which case it wouldn't need power from MIDI.

If you have some device that must be grounded through its input connector shell, you could wire up your own cable that ties the shell at that end to pin 2 on the other end, and plug that into the Polaris.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...