MIDI Line Driver over Cat5

[alauth]

Hello everyone !

My first time posting a thread here, hope I won't break any rule ! (don't hesitate to tell me )


I play in a band and we use MIDI to control our pedalboards. My goal is to create a solution to run MIDI over more than the 15m limit.

I saw this schematic about building a MIDI Line Driver over Cat 5 : https://www.edn.com/send-midi-signals-over-long-distances/

So I recreated the schematic in EasyEDA and worked on the PCB to make it work.


Four questions :

- Do you think I did anything wrong in my PCB or in my schematic ?

- On the schematic, I don't really understand the role of R3. Is it some kind of biasing of the signal between the 4N25 optocoupler and the input from the 74LS07 ?

- LED receive and send are supposed to show MIDI Data activity but I don't understand the setup with the +5V input on one side of the LED and the 74LS07 on the other side, could someone explain this to me ? (or point me to the right direction to understand this by myself ?)

- On the original schematic, there's C4, a 100nF cap between +5V and Ground, I thought it was just for stability improvement but with my power supply design, I think I don't need it anymore, am I right ?


My questions are probably quite basic but I'm learning all by myself so obvious things are sometimes not that obvious for me


Have a nice day everyone, thanks !

Arthur

 

[guy_4]

Hello alauth,

The 15 meters limit for MIDI link is an urban legend.
I have experience at running much longer MIDI links with no issue, " no data loss or timing delay " ....
Buy a good quality pair cable, 50 meters, and do your own test first.

Best,
Guy

 

[gyraf]

Agree, MIDI works with almost anything you throw at it

Technically it's just a simple 31KHz serial data stream, optically isolated, no handshake. Very far from today's near-GHz and handshake -based USBprotokols - these require an answer from the other end within a handful of nS, making length matter a lot.

I've used din/xlr converters to get through a 75m stage/mixer cable

 

[alauth]

I'll test that ! Thank you guys for your answer

But I'm still interested in knowing if what I designed is right, if you guys have time to have a look at my schematic and PCB

 

[zamproject]

As already stated, MIDI is low speed, opto-coupleted current loop, asynchronous, which make it a strong signal.

Design seem ok, which is just a buffer and a sym Tx/Rx IC.
I suppose it could be doubled to offer 4 lines on a single cable which to me is the only benefit of this.

But I'm not sure about the 5V within the CAT5 line ?!? is that to offer supply from one side only ? then I'll be concerned about that more than the signal for long distance, current is low but cable is thin, voltage drop may be calculated for long length ?

For really long distance and network abilities, RTPmidi is an option...

 

[alauth]

Yeah, it would be for supplying power to the second unit ! You think on a, say, 30m length, voltage drop would be an issue ?

 

[zamproject]

I don't know what is the overall consumption... 60mA for the status LEDs, about 2x50mA for line TxRx (not sure about it in datasheet)+ buffer ?
So saying 200mA...for 30m, AWG24 @ 25°C...voltage drop is 514mV

You should double check the math, and see how will behave IC with that in real life.
Knowing that Line Tx and Rx -recommanded operating condition- give Vcc min @4.75V... possible issue...

Now if you have extra wire/pair available on the CAT5 you can lower that, with 2 pair for 5V you get 1/4, about 125mV

Another questioning is the shared 0V and shield with your single -GND- label/net ? some may have answer about it.

The initial MIDI current loop wont suffer from this, as everybody say before just test with a quality mic cable before jumping on a design that request extra stage box, power supply, more connector junction, and time to build...

 

[alauth]

Maybe I could send the +9V down the Cat5 instead of the +5V ? This way, each unit would have regulated +5V but I'd only need one 9V power supply ?

And I could use two different lines from the Ca5 for the power supply.

Would it be a good improvement ?