I/O Map
The following table lists the main I/O definitions of the FRED 1.5. Next to the I/O below there was also an extension made to add RCA Video Coin Arcade controls with the same I/O as that machine.
| EF1 | Group 1: hex keypad or punch card data ready Group 3: tape data ready |
| EF2 | Tape running / stopped |
| EF4 | Tape error |
| OUT 1 | Set I/O group |
| OUT 2 | Group 1: 1 = program mode, 2 = direct mode Group 2: display type; 0 = TV off, 1 = 32x32, 2 = 64x16, 3 = 64x32 Group 3: bit 4 = program mode, bit 5 = direct mode, bit 6 = write mode |
| OUT 3 | Group 3: bit 0 = 1 - run tape, bit 1 = 1 - sound on, bit 2 = sound |
| INP 0 |
Group 1: read byte from hex keypad or punch card Group 3: read byte from tape |