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Showing content with the highest reputation on 10/06/2016 in all areas

  1. I've just come across this thread, and seen a few of the ideas. First thought: MIDI is a current loop interface, specified at 5mA loop current, though often running a bit higher to give leeway for differing opto isolator input LEDs. The voltage is not really the issue. Changing the input of the 9090 is a bodge, it works fine with other sequencers - you will only get the same problem with other devices, especially those using older optos like the CNY series. The simplest solution to me, would be to reduce the value of the output resistors in the sequencer. Since there will always be a 220R in the input circuit of the target MIDI device, dropping the resistors at the sequencer end to 100R or 68R would get closer to the spec, and the max, shorted output current from the sequencer , (in case of a bad lead) would still be a fairly safe 16 - 25 mA. (standard short circuit current on 5V would be a round 11ma). Oh, and if anybody is thinking of termination impedance matching: try to remember this is low KHz speed, not RF, and the lower output impedance would actually help keep the pulses square.... (added 05/11/2017) The latest MIDI spec, (CA-033) MIDI 1.0 Electrical Specification Update [2014], suggests 33 Ohms from pin 4 to the 3.3 V rail, and 10 Ohms from the device output to pin 5. This makes for a short circuit current of 76mA, which I consider too high for safety on a microprocessor output. I think it's good practise to use an output buffer: if you are working live, a spare buffer chip can be kept in the toolkit. A spare micro module is not so easy. Just a few thoughts anyway Mike
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