ZX-Spectrum +3's Auxiliary interface

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