I've had my SlimPhatty for a good while and have just done my best to ignore it's midi problems.
A standard midi cable stays plugged into the Midi IN connecting my gear sending midi, no usb.
When I turn on the power initially the SlimPhatty does not respond to any data.
I have to unplug the midi cable first, turn off the power and turn it back on and wait for it to completely do it's power up routine. Then I can plug the midi cable back in and it works.
Occasionally when using it, the note on and off response becomes reversed.
A note on makes the Phatty sound like a note is being held, and a note on makes it's sound cut off.
When this happens I have to do the same thing as when I start it up.
Unplug the midi cable, power cycle and wait for it to completely start up, then plug the midi cable in.
This is a real PITA even with the back of the Phatty slightly accessible in the back.
This is my second SlimPhatty in the last 8-10 years.
My first one would do the same note on/off reversal, and I would resolve it the same way.
Yet it would power up and work most of time with a midi cable plugged in.
I had completely different gear then.
I bought my current one a ew years ago and it's doing similar things, but ALWAYS with the power up routine.
None of these things happen if I were to use the usb cable.
My entire rig is generally computer-less.
Am I doing something wrong, or do Slim Phattys just struggle greatly when hooked up with a classic midi cable?