Hi All,
I've been wondering about this for years...
 
If I have an instrument with standard MIDI connectors (you know, those DIN connectors) is it safe to connect/disconnect them while the computer is powered up? I've read that it's best to have the instrument itself powered off, but I don't recall any mention of the computer.
 

I don't make a practice of that but have never worried about it either.  I'm sure I've midi-plugged unplugged computer and synth lots of time.  Never thought about.  Don't make a habit of it, tho.
 
@

Midi uses optical isolators to avoid introducing ground loops so the answer is yes you should be fine plugging cables in or unplugging with power on

I've been continuing my own research as well, so now I'm confused. I read today (http://www.dannychesnut.com/Music/Yamaha/Clavinova/Information/ComputerConnect/ClaviPC_E.pdf) that both computer and MIDI device need to be off before making connections. Or is this peculiar to the Yamaha Clavinova? Or maybe because the manual linked to here was written in 2000?

I'm not sure why, but if I power up my StudioLogic midi keyboard while a Sonar project is open, some of the softsynth settings suddenly revert to default values.
 

I normally plug/unplug MIDI cables while powered down, but I have done it in & out of various MIDI modules, keyboards & my hardware sequencer while on, because shutting everything down just seemed like to much trouble. [1 keyboard, 6 MIDI modules, 1 sequencer - then having to press dozens of buttons and reassign and/or load things again]
 
As for that article, that's 14 years old now, and although MIDI had been used for a while by then already, it was still foreign to many & still coming out of it's infancy compared to today, for some, so that 'Caution' note on page 7 might have been simply stating precautions for use more than anything else.
 
Virtually all manuals & instructions will suggest having things turned off before any connections are to be made - standard policy from everything I've seen over the years. [Mainly for law suits and legal ass-coverage more than any other reason.]
 
But even though it's nominal [minimal] voltage connections that you're making & breaking, it is at least some amount of voltage nonetheless, so probably best not to do it 'hot' if at all possible.

@ jimusic: Thanks. I was hoping it was all a thing of the past because, as you said, it's a big pain. It's not like the Commodore 64 where cycling the power takes less than two seconds.