PURPOSE: To enable even a 16-bit CPU to access an option board connected to an 8-bit CPU by providing a timing indicating means which indicates the timing when an address signal and a data signal are separated from each other by a separating means.

CONSTITUTION: This data bus compatible system where CPUs different in bit constitution like an 8-bit arithmetic processing unit CPU and a 16-bit are used together is provided with a separating means which separates the multiplexed signal of the address signal and the data signal into the address signal and the data signal and a timing indicating means 7 which indicates the timing of this separation in the separating means 2. For example, multiplexed address signal and data signal outputted from the 16-bit CPU are separated in parallel by the separating means at the timing indicated by the timing indicating means, and therefore, data can be sent to the 8-bit device.