Bi-directional Midi fun

Other Hardware General Electronics
1

Hello,

I just picked up an old A.R.T. X-15 Midi controller and an SGX2000E (already got an awesome Savatage tone out of it in the two days I've had it!), and of course, I must modify it.
The Midi controller is a pedalboard style, and it has Midi in, out, and thru. Also, it has a power port. (duh)

Now, my goal is to fit Midi in, out, and power onto a regular 5 pin midi cable so I can wire the X-15 to the processor with one wire, instead of 3. I don't want to use a 7 pin cable, otherwise I will need backups of those, whereas all my other Midi stuff uses 5 pin cables.

I was looking at this general schematic for Midi, and I noticed that there is a (source?) and a (sink?) for the input, and the output looks like a single line and a 5v+ output.
I'm assuming it's sort of a remote switching mechanism?
Could someone tell me if I could wire the 5 pin cord to be setup as Midi input, output, + and - power, summing up to 4 pins used, then having an Arduino on either end to do the "switching" that I stated above, whilst providing power to the pedalboard? Or is the Midi input a Clock & Data style setup? If its the first, then I suppose I'm basically doing uart at the Midi baud-rate.

One other question, the 5 pin midi cord, is it actually 5 conductors, or is it 6 (5 isolated wires and a bare wire touching the shielded foil) ?
I would assume it is the latter, and that would be why a lot of devices have the ground lift if their chassis is made of metal. But you know what they say when you assume.. :slight_smile:

Thanks!
-Matt

2

It's not clock and data, I know that much. It's straight async serial at 31250 baud.

In the MIDI cable only two are active (as per your diagram) and the earth is not connected at one end to avoid ground loops.

I don't know that you are going to fit MIDI in/out and power into 5 conductors. I doubt it, but someone else may be better able to advise.

It seems to me you would need two conductors each for in and out (since they don't use ground) and that doesn't leave much over for power and ground.

3

MattiasOfTheMetal:
I was looking at this general schematic for Midi, and I noticed that there is a (source?) and a (sink?) for the input, and the output looks like a single line and a 5v+ output.
I'm assuming it's sort of a remote switching mechanism?

I think in fact MIDI is current loop system, see this page which confirms it:

It is the current flowing which activates the opto-isolator. I tried measuring the voltage once with a scope and it was all over the place, from noise (eg. lots of 50 Hz hum) but it was the current that makes the LEDs in the opto-isolator fire, and get converted into serial data at the receiving end.

4

Yes in a lot of MIDI circuits the 5V component of the line is connected through a resistor. This makes it safe when shorts happen. So basically you are one wire light of having a soloution.

5

Thanks for a fast response guys!

I verified that a 5 pin midi cable is only 5 conductors, and I believe the middle conductor (2) is the bare wire that touches the foil shield on the inside of the cable.
5 wires plus a 6th bare wire that touches the foil would have been nice (the bare wire would connect to the plug shell on either end of the cable).

In the schematic I linked above, pin 4 is a hot signal, and pin 5 is switched to ground at the Midi message's values.
If I were to combine in and out in a single cable, I think I can do 3 pin Midi instead of 4. Basically, making in and out share a 5v line, and the two message lines will switch a transistor. Then I have two more pins for + and - power.

Seem doable? Or am I missing something?

-Matt

EDIT:
I think I figured it out. On the master (processor) pin 1 add: 5v or 9v to power other stuff in the slave (pedal), pin 2 remain: ground, pin 3 add: Midi out, pin 4 remain: Midi in 5v line that can share with the 5v line for Midi out on the slave, pin 5 remain: Midi out (signal).
That way it is still backwards compatible with regular two cable midi devices. I was planning to add this to the Midi in on the master, and Midi out on the slave, so I don't have to drill more holes.

EDIT 2:
In the first schematic I linked to:

Could pin 4 on both the Midi in and out ports on the same device, be connected together? It would still be the 5v+ -> 220?(out port) -> 220?(in port) -> Midi goodies; instead of the 5v from the other device plugged into this one, couldn't the power just be ported accross both pin 4's without damaging it? What is the point of isolating the 5v supplies between two devices like this?

6

Could pin 4 on both the Midi in and out ports on the same device, be connected together?

No.
The point of the MIDI standard is that the two units do not have to share a common ground, therefore they can't share a common +5V.
I am not saying this won't work with some devices but it will not work with all devices.