Connecting a MIDI Interface to an SGI O2

I use MIDI on my O2 very reliably.  You need to convert the serial ports to RS422 and wire a special interface cable.

I. Converting the O2 RS232 serial ports to RS-422

I use an Antona RS232<->RS422 converter and a Studio 3 MIDI interface. The Antona unit is a small in-line DB9 to DB9 adapter which plugs directly into the serial port. It can be used parasitically or supplied with external power. I use it without external power, and simply wired the TX and RX signals and ground to a miniDIN-8 connector on the other end of a 20' cable. I did have problems using the unpowered converter with an unpowered MIDI interface like the Translator.

The part is the Antona ANC-6090, $49.00 US.

Antona Corporation

II. Wiring the cable

Here is the wiring I use for the cable:

MiniDIN8    DB9     Signal
1           N/C
2           N/C
3           8       TX-
4           5       GND
5           2       RC-
6           3       TX+
7           N/C
8           7       RC+
N/C = not connected
The cable shielding is also connected on both ends.

Pin numbering (looking down from the outside end of the connector toward the pins):
MiniDIN 8:
  >  1   2  <
 /           \
 |           |
 | 3    4  5 |
 |           |
 \           / 
  \ 6  7  8 /
\ 1   2   3   4   5 /
 \                 /
  \ 6   7   8   9 /

You can also refer to the pin diagrams in the online hardware documention.

If you have this exact wiring, connect the DB9 to the Antona and the Antona directly to the O2 serial port. It should then work with a powered Macintosh MIDI interface. Unpowered MIDI interfaces may not work (although I haven't tried supplying external power to the Antona converter). It works very reliably with my Opcode Studio 3. If you still have trouble make sure that you have the serial port configured correctly for MIDI (check the Serial Device Manager).  I am told that using Serial Port 2 is more reliable than Serial Port 1, although I have used both.

Michael McNabb

Last modified: Fri Jun 26 18:03:48 PDT 1998