Further suppose that the 8051 microcontroller 170a desires to communicate with batteries or fans or other components attached to the I2 C bus 149 it first examines the expansion base ready signal ERDY. If the signal ERDY is active (at a logic low level), the 8051 microcontroller 170a recognizes the expansion base E is communicating over the I2 C bus 149 and does not interrupt it.