MIDI output voltage

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barry

Joined: 31 Mar 2005
Posts: 23
Helped: 1

16 Jun 2009 1:48 midi voltage

Does anybody know the specification for the MAXIMUM output voltage a MIDI driver will see?

The output is generally an open-collector pulling a resistor to ground, so that nominally 5mA flows, but I don't know what the maximum voltage that could be applied to the other end of that resistor might be. (And I'm too cheap to buy the spec just for this one bit of information). Older designs I've seen use 7406's that are good up to 15 volts. I'm working on a new design, and want to know if I can use a low-voltage(3.3V) open-drain device (LCX, LVT, etc.). These devices can be 5-V tolerant, but not much more.

Thanks,
Barry

nico_reloaded

Joined: 16 Jun 2009
Posts: 5
Location: Argentina

19 Jun 2009 19:46 mdi voltage

Hello barry!
Voltage doesn't matter in MIDI, only the current you can drive is important. Generally you have a led from an optocoupler at the other side, so you have to give it enough current to turn this led on. Hope i've been usefull (sorry about my english!)

Regards

Nicolás

barry

Joined: 31 Mar 2005
Posts: 23
Helped: 1

19 Jun 2009 20:01 midi to voltage

Actually, the voltage DOES matter in the sense that there is a limit to what the driver can withstand when it's off. 10 Million volts across a 2 billion ohm resistor will still give 5 mA, but when that transistor turns off...

There MUST be a spec for what the maximum voltage can be.

P.S. Your English is fine! (Much better than my Espanol)

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FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

19 Jun 2009 21:37 midi to voltage

Quote:

but I don't know what the maximum voltage that could be applied to the other end of that resistor might be.

Please look at a MIDI circuit. The voltage is supplied from the driver side, and it's ususally 5V. The receiver side is passive, simply an opto coupler LED as already said.

barry

Joined: 31 Mar 2005
Posts: 23
Helped: 1

20 Jun 2009 17:41 mdi voltage drive

It's that "usually" that has me concerned. Could it be 15 volts? There HAS to be a specification, or how can you size the resistor on the driver side? I've seen 220 ohms used, which would give about 10 mA with a 5V supply. But 10 volts would give 20 mA, which is still within the acceptable range of most optoisolators, but would damage a 74LCX06.

FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

20 Jun 2009 21:58 midi circuit opto

I don't understand the problem. You are defining the MIDI voltage at the driver side. I said usually, because I'm not absolutely sure, if there are any MIDI drivers, that use a higher supply voltage,

But I can be sure, that my MIDI circuit uses a 5V supply.

barry

Joined: 31 Mar 2005
Posts: 23
Helped: 1

23 Jun 2009 19:18 current driven midi out

I'm not defining the MIDI voltage, that's supplied at the receiver end. All I provide is an open-collector output that connects to the cathode of an LED in opto-isolator sits in the OTHER instrument. The Anode of that LED is tied to the voltage in question.

FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

23 Jun 2009 23:59 midi out open collector

I'm refering to the wellknown MIDI 1.0 standard, that uses a 5 pin DIN 180° connector. It clearly involves a
current loop, which means that the receiver is passive and the loop is supplied from the transmitter. I see two
possible reasons, why you claim that it's different in your case:

- You are using a different MIDI physical layer, that I don't know yet
- You didn't understand the MIDI 1.0 circuit

barry

Joined: 31 Mar 2005
Posts: 23
Helped: 1

25 Jun 2009 1:43 midi low voltage problem

Okay. I'm an idiot. You're right; I didn't realize (or forgot) that the transmitter side supplies the voltage to the optoisolator LED. Thanks for helping me see the (LED) light.

Barry