---

John's Port-Powered Midi Interface for

PC Soundcards

(Breakout Box)

I have built one of these myself, exactly as described, so I know this design works. However, I make no promises that you will be able to do likewise, or that I will be able to help you if yours doesn't work. I don't mind trying to help if you contact me, and most people who have done so have managed to get their interface going.

It is also worth noting that Jim Rowe (of Electronics Australia and more recently with Silicon Chip - both Australian electronics magazines) has published two versions of a similar circuit. These are available (by mail order if required) as kits from a number of Australian electronics stores. Dick Smith Electronics may still have the original version 'MIDI Breakout Box' kit. It may even be cheap (with limited stock) as it's been superceded. The newer version, 'MIDI Mate' is a far neater design with a very nice box and front label (in my opinion). Jaycar Electronics has the full kit (all hardware including printed circuit board) for this project for AUD$39.95 and it can be ordered from them by email. Dick Smith will probably also have the later version as well although I haven't checked this. (Please note: I have absolutely no affiliation with Dick Smith or Jaycar). Important note: I recently built one of these 'MIDI Mate' kits for a friend. The MIDI In port did not work reliably. Neither of these kits have a resister from pin 7 of the optocoupler to ground. The addition of this resistor to the board fixed the problem (the reason for my inclusion of this resistor in all my designs is mentioned in the Mac version)

I also take absolutely no responsibility for computer or midi equipment which might be damaged during your efforts to build this device and get it working. It is theoretically possible for this to occur, especially if you connect the wires to the computer incorrectly, although it appears to be unlikely as none of the many people who have contacted me have run into any problems so far.

Most PC Soundcards have midi capability these days and the connections are provided via the joystick port. You still need an interface, however, but because part of the 'interface' is already provided in the soundcard the circuit I am about to describe is often referred to as a 'breakout box'.

It became obvious after some reading that with a few modifications to my previous Mac interface design, a suitable circuit for a PC could even more easily be built. There is no need for stealing power from the data transmission lines (as there is with the Mac) since +5V is supplied by the soundcard. There is also no need for an external clock circuit. Deleting these sections, changing a few links and resistor values, and using the extra buffers for a 'thru', I came up with the following circuit which worked first go!

Schematic

Board Layout (component side)

(Click on the picture for a larger version to print separately)

Thanks to Marnix Arnold of the Netherlands for pointing out the link that was missing from pin 4 to pin 7 of the 4049! The picture above is now CORRECT (as of March 2000)

A = Joystick Port Pin 8 (+5V) {Pins 1 and 9 are also suitable}
B = Joystick Port Pin 5 (0V) {Pin 4 is also suitable}
C = Joystick Port Pin 15 (soundcard midi in)
D = Joystick Port Pin 12 (soundcard midi out)

Note: Pins 7, 9 and 11 of the 4049 are linked by bridging the copper tracks under the board, as are pins 2 and 14.

Parts List

R1: 220R
R2: 220R
R3: 10K
R4: 470R
R5-11: 220R

C1: 10uF 10V tantalum

D1: 1N914 (or 1N4148)

U1: 4049 hex inverting buffer
U2: 6N138 or 6N139 optoisolator

J1-5: 5 pin DIN PCmount

Misc: box, cable and 15 pin 'D' plug (and socket?) for joystick port (see tips below)

Board (copper side)

Tips

There are several things you might want to consider from a hardware point of view:

• If you build the thing with the five midi sockets as shown above, you might have a hard time finding a box to fit exactly since there are sockets on both sides of the board. One solution is to use chasis mount sockets instead of PC mounted ones as I did but this requires separate wiring to each socket which is a bit less tidy. Another solution would be to increase the number of strips (width) of your stripboard so that the sockets fit exactly within your box. This would obviously require you to increase the lengths of the link and resistor (R8) to J5 which is no big deal. Yet another solution is to forget about the third Out and use that socket for the Thru.
  
  
• It is possible to include a 15 pin 'D' socket as well to avoid the need for swapping midi interface and joystick all the time. It can be wired straight through and as far as I can tell the interface should not interfere with the function of your joystick. The best way to do this is to use 15 conductor ribbon cable and IDC type plug and socket (no soldering required). The plug is put on one end of the cable (to go to the soundcard) but the socket should be put about 10-15cm in from the other end. The 4 required wires can then be separated from this 'overhanging' bit to go to the circuit board and the other 11 wires trimmed off at the socket which is then mounted somewhere on the box for you to plug your joystick into. If you use IDC plug/socket make sure you get the order of the wires correct!
  
  
• Strictly speaking, all the midi Out and Thru ports (not the In port!) should have both the shield and pin 2 connected to ground. This does not appear to be absolutely necessary and I haven't really made provision in the design above to do so but you can if you want.
  
  
• Please also take a look at the interface page for the Mac. There are some technical and hardware details in that page (eg the interchangeability of chips and so on) which apply to this PC version as well.

To John Loadsman's page