Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site redwood.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!sdcsvax!sdcrdcf!hplabs!hpda!fortune!redwood!rpw3 From: rpw3@redwood.UUCP (Rob Warnock) Newsgroups: net.music.synth Subject: Re: Re: MIDI Message-ID: <182@redwood.UUCP> Date: Sun, 3-Mar-85 06:46:22 EST Article-I.D.: redwood.182 Posted: Sun Mar 3 06:46:22 1985 Date-Received: Thu, 7-Mar-85 04:01:47 EST References: <3416@alice.UUCP> <8617@brl-tgr.ARPA> <102@cvl.UUCP> Organization: [Consultant], Foster City, CA Lines: 59 +--------------- | > > 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