"Roy J. Tellason" wrote to "MIKE ROSS" (02 Feb 03  20:01:43)
 --- on the topic of "Monitors"

 "Roy J. Tellason" wrote to "Jasen Betts" (02 Feb 03 
04:06:16)  ---
 on the topic of "Monitors"
 JB> MIDI is also balanced,

 RJT> No it's not.  Typical driver circuit is one resistor to +5 and another
 RJT> tied to the output of an open-collector TTL gate.

 MR> MIDI is 5mA current loop, logic 0 is current ON.

 RJT> Right.  But it's still not balanced...

I didn't say it was, Jason did. I was only pointing out it was a current
loop similar to the old IBM 20mA spec, except it is 5mA.


 JB> but AFAIK most midi line driver circuits arent up to driving the
 JB> signals long distances.

 RJT> Yeah,  a couple of 220 ohm resistors at the driving end and one more
 RJT> at the receiving end don't give much margin when run off a +5
 RJT> supply...

 MR> MIDI spec is 50 ft max but 100 ft is not unusual.

 RJT> So they used 74xx in stead of 74LSxx parts on the driving end,  or
 RJT> CMOS instead of 74LS on the receiving end...

Nothing special done, just regular 74LS jelly beans. Also note that each
MIDI IN usually has a MIDI THRU which is a repeater so that more boxes
can be daisy-chained for even longer distances. Of course the problem
may then arise having to deal with delays. For example some MIDI
OUT's/THRU's mix the local data with the received data so that there are
processing delays involved. A normal THRU however has very little delay
other than the opto-coupler/gate and the negligible 1ns every 9 inches
through space or wire conductor. When delays get into the 1ms range then
it's troublesome. Kind of like a beginning pianist where a chord will
sound with a tiny bit of delay between each tone. It's usually not that
bad however.

 Mike
 ****

... Just wondering if you are still out there in Fido land
--- Blue Wave/DOS v2.30