[sdiy] Midi powered?

[vintvolt]

Hi all!
I noticed that some Midi devices are able to power small external 
devices while others can't. For example, my roland PC-180 Midi 
controler can power a small midi merger just by connecting its midi OUT 
to the mergers IN. Other devices connected the same way won't turn on 
the merger (Anatek pocket merge), for example a doepfer fader-box or 
the korg electribe IIRC
I digged through the midi specification, but couldn't find any 
reference about this. Is this an undocumented feature adopted by some 
manufacturers? Could they possibly use one of the unused pins of the 
connector? Is this legal?
PS: The anatek merger does not have a separate power supply connector, 
so it relies completely on power over the midi cable. Maybe it's just a 
matter of current drive capacity? Some devices provide more mA at the 
OUT port, so they are able to drive not only the optocoupler, but other 
chips as well?

Thanks!