I am working on a project using a Teensy 3.1 that will use a 5-Pin Midi output and would like some advice on the best way to build the circuit. As you know, the resistance of the Midi output circuit must be modified to work with 3.3v. The Teensy website advises to replace the two 220 Ohm resistors with 47 Ohm ones. We add this to the 220 Ohm resistor that is in the Midi input of the other device we get a total resistance of (47 + 47 + 220 = 314 Ohms) for the current loop.
But, according to the official MIDI 1.0 Electrical Specification Update located below, the resistors should be replaced with a 33 ohm and 10 Ohm values which would give a total resistance of (33 + 10 + 220 = 263 Ohms).
https://mitxela.com/other/ca33.pdf
The reasoning given in the Midi document is this: "The opto-isolators found in some MIDI devices have an LED maximum forward voltage drop of 1.9V and can require up to 5mA typical current drive for normal operation."
If we take their example of a Forward Voltage (Vf) of 1.9v, our current would be:
- Teensy site resistor values: (3.3v - 1.9v) / 314 Ohms = 4.5 mA
- Midi site resistor values: (3.3v - 1.9v) / 263 Ohms = 5.3 mA
As you can see, the Teensy site values fall below the suggested drive level and potentially cause intermittent dropouts.
If we go to the other extreme and use a Vf of 1.1v (found as the minimum value for some opto-isolators) we would get the following:
- Teensy site resistor values: (3.3v - 1.1v) / 314 Ohms = 7 mA
- Midi site resistor values: (3.3v - 1.1v) / 263 Ohms = 8.4 mA
Given that most opto-isolators can handle a max current of 20 - 40 mA is there any reason not to use the Midi Site values?
Please let me know if I am missing something.
Cheers.
But, according to the official MIDI 1.0 Electrical Specification Update located below, the resistors should be replaced with a 33 ohm and 10 Ohm values which would give a total resistance of (33 + 10 + 220 = 263 Ohms).
https://mitxela.com/other/ca33.pdf
The reasoning given in the Midi document is this: "The opto-isolators found in some MIDI devices have an LED maximum forward voltage drop of 1.9V and can require up to 5mA typical current drive for normal operation."
If we take their example of a Forward Voltage (Vf) of 1.9v, our current would be:
- Teensy site resistor values: (3.3v - 1.9v) / 314 Ohms = 4.5 mA
- Midi site resistor values: (3.3v - 1.9v) / 263 Ohms = 5.3 mA
As you can see, the Teensy site values fall below the suggested drive level and potentially cause intermittent dropouts.
If we go to the other extreme and use a Vf of 1.1v (found as the minimum value for some opto-isolators) we would get the following:
- Teensy site resistor values: (3.3v - 1.1v) / 314 Ohms = 7 mA
- Midi site resistor values: (3.3v - 1.1v) / 263 Ohms = 8.4 mA
Given that most opto-isolators can handle a max current of 20 - 40 mA is there any reason not to use the Midi Site values?
Please let me know if I am missing something.
Cheers.
