Of the 5 pins in a MIDI DIN, 3 of them are used for MIDI Out and Thru. The center pin is Ground, one pin is the UART for transmitting and receiving data, and the last pin is a +5V current.
DSI instruments don't provide the +5V current, so they won't power MIDI-powered devices.
Why would the Father Of MIDI not follow the MIDI specification to the letter? I've wondered that for years. Hopefully the device manufacturer provided an alternate means to power the thing.
My Little Phatty doesn't provide +5V, either, so it's not that uncommon. My guess (and now we're in speculation territory) is that the center pin isn't connected to Ground like it should be for the MIDI Out...?