The AUX (auxiliary interface) socket supports two input lines (pins 2 and 5) and two output lines (pins 2 and 4). The I/O lines are driven by 1488 and 1489 line driver chips which are, in turn, connected to the I/O lines of the AY-3-8912 (see the manufacturer's data sheet for this device). Basically, register 16 of the AY-3-8912 controls eight I/O lines, the bits are designated as follows...
| BIT | SIGNAL |
| 0 | AUX pin 2 (out) |
| 1 | AUX pin 4 (out) |
| 2 | RS232 pin 5 (CTS out) |
| 3 | RS232 pin 3 (RXD out) |
| 4 | AUX pin 3 (in) |
| 5 | AUX pin 5 (in) |
| 6 | RS232 pin 4 (DTR in) |
| 7 | RS232 pin 5 (TXD in) |
Using software control loops, the I/O lines could be driven as a second RS232 port (in the same way as the RS232/MIDI socket is driven using bits 2, 3, 6 and 7). Alternatively, the I/O lines could be used to drive, for example, a robot or some other external device.
___________________________
__| 6 5 4 3 2 1 |
|__ ### ### ### ### ### ### |
|___________________________|
AUX socket
| PIN | FUNCTION |
| 1 | GND |
| 2 | OUTPUT BIT 0 |
| 3 | INPUT BIT 4 |
| 4 | OUTPUT BIT 1 |
| 5 | INPUT BIT 5 |
| 6 | +12V |