Casio 120CR Service Manual page 13

[Data reading procedure]
When the CPU reads the data of EEPROM, CPU sends the CS signal and SK signal to EEPROM.
Then, CPU sends the address to EEPROM, and then EEPROM sends the data to CPU.
CS
SK
DI
DO
[Data writing procedure]
When the CPU saves the data to EEPROM, CPU sends the CS and SK signal to EEPROM.
And CPU sends the address and data to EEPROM from pin No.15.
When the EEPROM writes the data , EEPROM outputs the busy signal from DO terminal to CPU.
After finish writing, EEPROM outputs the ready signal from DO terminal to CPU.
CS
SK
DI
DO
High Impedance
Output data here
Address
— 11 —
Address
Data
Status
Data
BUSY(L)
READY(H)