I/O Map
The following table lists the main I/O definitions of the HUG1802 and ETI-660.
| Q |
- Sound on/off - Cassette out |
| EF1 | CDP 1864 display: display status |
| EF2 | Cassette In |
| EF4 | Step key pressed |
| OUT 1 | CDP 1864 display: step background colour and display on |
| OUT 2 | Write PIA MC6821 |
| OUT 3 | Write colour RAM |
| OUT 4 | Tone latch, which sets tone frequency |
| INP 1 | CDP 1864 display: display on |
| INP 2 | Read PIA MC6821 |
| INP 4 | CDP 1864 display: display off |