kahrs@alice.UUCP (Mark Kahrs) (02/23/85)
With all these postings about MIDI and RS-232 I thought I'd put in my $0.03 worth. First off, the clock rate is 31.25 kBAUD. Now, all you RS-232 fans out there may recognize the fact that 31.25 KB ain't exactly standard. Right. As Dave Sewhuk pointed out, UARTs take a 16x clock, so 16 * 31.25 gives you 500 Khz. So, some person had this extra 500 Khz clock and decided to foist it on the rest of us. So much for straight conversion. Also, as Dave Hsu pointed out, the MIDI standard requires a current loop interface, hence an opto- isolator. Now, it's definitely true that Roland makes the MPU-401 which enables a micro to talk MIDI but their documentation leaves MUCH to be desired... Another possibility is to use the Mac output port with some external circuitry and an external clock. Anyhoo, hope that helps someone. I'm delighted to see this group! Cheers, Mark.
ron@brl-tgr.ARPA (Ron Natalie <ron>) (02/25/85)
> Also, as Dave Hsu pointed out, > the MIDI standard requires a current loop interface, hence an opto- > isolator. > Eh? The schematics for the MPU show the lines to be balanced pairs. They are driven by convetional drivers, no optoisolaters are involved. -Ron
hsu@cvl.UUCP (Dave Hsu) (02/26/85)
> > Also, as Dave Hsu pointed out, > > the MIDI standard requires a current loop interface, hence an opto- > > isolator. > > > > Eh? The schematics for the MPU show the lines to be balanced pairs. > They are driven by convetional drivers, no optoisolaters are involved. > Most odd. Having seen several schematics and a number of Apple/Midi interfaces, there are most certainly opto-isolators involved. And my ref. books describe to the hook-ups as current-loop interfaces. I'll dig up exact references by Wednesday. =Dave Hsu= (301) 454-4526 ARPA: hsu@cvl hsu@umd2 USnail: Computer Vision Laboratory CSNET: hsu@cvl Center for Automation Research BITNET: hsu@umd2 University of Maryland UUCP: {seismo,allegra,brl-bmd}!umcp-cs!cvl!hsu College Park, MD 20742
dave@rocksvax.UUCP (02/27/85)
Opto-isolators are the best things to use when you want to electically
isolate signals. Most musical equipment tends not to get grounded very
well and you can end up with a healthy current flow through the grounds
on your signal lines. These lines tend to be small gauge and can easily
build up volts of ground loop messing up the signal, not to mention the
potential of electrocuting the musician, and/or frying the instrument.
While not strictly necessary a side benefit is it easily converts currents
to voltages and give you about 2000V of isolation in a little package. You
could always use a plain op-amp as a current to voltage converter, but
I suspect that it costs the same The opto way giving you as a bonus this
isolation benefit.
Dave
arpa: Sewhuk.HENR@Xerox.ARPA
uucp: {allegra,rochester,amd,sunybcs}!rocksvax!davehsu@cvl.UUCP (Dave Hsu) (02/28/85)
> > Also, as Dave Hsu pointed out, > > the MIDI standard requires a current loop interface, hence an opto- > > isolator. > > > > Eh? The schematics for the MPU show the lines to be balanced pairs. > They are driven by convetional drivers, no optoisolaters are involved. > > -Ron Refer to David Dronan's Exploring MIDI: the Musical Instrument Digital Interface. The spec calls for a 5ma current loop interface. How the Macintosh can accomplish an interface with NO MORE THAN A CABLE would be my next question. How is the Mac capable of handling current loops? -dave
rpw3@redwood.UUCP (Rob Warnock) (03/03/85)
+--------------- | > > the MIDI standard requires a current loop interface, hence an opto- | > > isolator. | > Eh? The schematics for the MPU show the lines to be balanced pairs. | > They are driven by convetional drivers, no optoisolaters are involved. | > -Ron | Refer to David Dronan's Exploring MIDI: the Musical Instrument Digital | Interface. The spec calls for a 5ma current loop interface. How the | Macintosh can accomplish an interface with NO MORE THAN A CABLE would be | my next question. How is the Mac capable of handling current loops? | -dave +--------------- Tutorial time on current loops, folks! (Having spent some years building current-loop communications equipment...) In any current-loop system, there must be a battery or "loop supply". The loop battery can be external to the all of the controllers (or interfaces) in which case all controllers must be floating (that is, opto-isolated), or it may be embedded in exactly ONE of the controllers, in which case that controller ONLY need NOT be "current loop", per se. Likewise, exactly ONE of the controllers can be connected to earth ground without causing "ground loops" and other hazards. Usually, that will be the loop battery or the controller which is acting as the "battery". (For example, your telephone is a "current loop", device with the battery at the central office, and grounded there.) The -422 interface of a Mac is capable of being the "battery", and can support a limited number of opto loop devices (given certain resistors, etc., in the cable), as long as the sum of the voltage drops (each opto receiver is about 1.5 volts and each transmitter about 0.2 volt) does not exceed the voltage drive of the -422 drivers in the Mac. Therefore, if the Mac is the "battery", IT need not have any opto isolators. (Clearly, TWO Macs may not be connected in this way to the same MIDI current loop.) Given that the -422 drivers do not break down, extra voltage could be supplied to the "battery" (say, with a resistor to +12) to extend the number of current-loop devices supported. There will be a finite (and probably small) limit on the number of controllers that can be in the loop. Teletype interfaces used to use quite high voltages, such as 60 to 100 volts, to extend the compliance of the loop. The target loop current was set with a large, high-power resister. These days, one uses "constant-current" FET devices, or op-amp circuits. Still, there must be enough voltage in (around) the loop to turn on all the opto receivers. (Note that the loop voltage must also not be so high as to break down the opto drivers in the other controllers!) I hope this has helped a little to explain the apparent contradiction. Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404
kevin@sun.uucp (Kevin Sheehan) (05/30/85)
<Without a keyboard, no-one can hear you scream> Having seen discussion of the MIDI interface, and having long exposure to the in's and out's of the electronics that go into a band, and the electronics that lead to this message, I am curious as to the definition of the MIDI interface. I will take the language it was written in, I will take a good working description, what I will not take is opinion, unless quite well put. I make this restriction due to the fact that there are quite a few of us here that are interested in what MIDI is.... And then, what it can do. pointers, real stuff, and tips appreciated. l & h, kev