Midi tx 3.3v !

C

clearskyer

Member
Hi just a small question ! When i set Usb midi interface_3x3 exemple, the TX pin on teensy 4.1 is sending 3.3v is this normal ? Im trying to get the info online but im a bit confused ! Thanks !
 
Yep but im mean the 3.3v comes from the data pin, that what seems off cause i though the voltage would come from the pin 4. Thank you!
 
Teensy Transmits and receives on ths serial ports, Serial1 being pins 0 & 1. Pin 4 is not used at all.
 
Sorry I mean pin 4 on the midi cable which would be the ' data' pin from what I understand. On the teensy all serial TX pins seem to send 3.3 volt with data. But when I compare to my controller or other synths on midi output, they dont send voltage on that pin.
 
clearskyer said:
Sorry I mean pin 4 on the midi cable which would be the ' data' pin from what I understand. On the teensy all serial TX pins seem to send 3.3 volt with data. But when I compare to my controller or other synths on midi output, they dont send voltage on that pin.
Click to expand...

If I am understanding what you are saying, then I believe that the schematic given <here> shows that MIDI OUT pin 4 is pulled up to 3.3VDC & pin 5 is actually DATA OUT.

Hope that helps . . .

Mark J Culross
KD5RXT
 
haha yeah lots of confusion again cause sometime with the orientation of the cable. But yeah, my question is: Is it normal, from that schematic you provided, pin 5 send 3.3v which comes from the pin 1 of the teensy which is the TX pin ( data pin ) on interface_3x3 sketch exemple ? Still when I plug it in my synth its working good but i find it particular :) Thanks you !
 
clearskyer said:
Is it normal, from that schematic you provided, pin 5 send 3.3v which comes from the pin 1 of the teensy which is the TX pin ( data pin )
Click to expand...

Yes, this is normal. "TTL level" serial signals are "active low". When idle, the pin is supposed to be logic high. The start bit is always low, the data bits are either high or low, and then the stop bit is always high.

Virtually all hardware serial since the early 1980s when MIDI 1.0 was published have worked this way, though back then almost everything used 5 volt signals. This is why the other pin on a MIDI connector uses a resistor to either 5V or 3.3V power. When the serial output is idle (logic high), the difference in voltage between the 2 pins on the MIDI connector is zero. Then when the serial signal goes logic low, the net output (if not connected) becomes either -5V or -3.3V. The receiver is always (or is supposed to be) another resistor and LED inside an optocoupler. So the LED is off when the signal is logic high and turns on when it's logic low.

Active low, where 0 means on and 1 means off, can seem counter intuitive. But it's actually pretty common and was almost universally used in the 1980s when most chips were either NMOS and or bipolar (mostly NPN) transistors. Those early chips simply didn't P-channel or PNP transistors for strong logic high output. CMOS became the norm in the early 1990s, many years after MIDI.
 
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Ok ! your confirmed what i was suspecting from a reading but wasn't sure ! ( didnt want to destroy some gear ) Thanks you so much ! My projet will be done soon and i'll post the final code ! Have a good day everyone :)
 
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