The RCA Studio II uses the 4 to 16 line decoder CD4515 to decode the 20 keys. The lower 4 bits of OUT 2 are used as input selecting one of the 16 outputs of the CD4515. 10 of the outputs are connected to both keypads (keys 0 to 9). The keys on the left keypad are connected to the EF3 flag and on the right keypad to the EF4 flag. The Studio II software checks the keys by stepping OUT 2 from 0 to 9. After every OUT, EF3 and EF4 are checked and if equal to 0 the key representing the last OUT 2 and EF was pressed.