Does anyone have figures on the capacitance per foot of those MIDI DIN cables? I've seen some schematics of MIDI Thru boxes that show the output for the cable being driven by a single 74HCT14 schmitt trigger section.
I can't say for certain, but my instinct tells me that perhaps this isn't enough drive for longish cable runs.
I did some Googling and found "specs" varying from 28 to 57 pF/ft. I measured 4 of mine and found the thin cables near the high end, and the fat cables near the low end.
I have one 50ft MIDI extension cable somewhere, that I made from 110 ohm digital audio cable (forgot the number) that was about 12 pF/ft. That was inspired by my purchase of a 50ft cable on eBay, which didn't work.
I've read about people using 300 ohm twinlead to make very long MIDI cables, but I have no experience. Of course, there are active MIDI cables with built in amplifiers for long runs.
A very rough guess(tm) of the maximum cazapitance is just the RC time constant. The above schematic shows a 220 ohm resistor between the driver and the cable. With a clock frequency of 31.25 KHz, I would want a bandwidth of at least 5 times to produce a square looking clock. That's about 150 KHz f = 1/(2*Pi*R*C) For a drive impedance of 220 ohm and 150 KHz, I get a maximum capacitance of 4800 pF. With my worst case crap cable of 57 pF/ft, that's about 84 ft max. I vaguely recall the spec is 50ft max and 5 microsec minimum rise time, but I couldn't find those anywhere in the MIDI specs.
Also, MIDI data is a 5ma current loop. The 75HCT14 claims 25ma maximum output current per section, but also 50 ma maximum supply current for the device. Seems possible, but I would be worried about excessive dissipation caused by slow rise/fall times. Try plugging into the equation on Pg 10 and see if it will get hot.
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