Device expanding MDIO interface through parallel bus

A device expanding an MDIO interface through a parallel bus comprises an MPU, a CPLD and a switch. The MPU comprises the parallel bus. The CPLD is a programmable device with the function that the parallel bus is converted to MDIO. The switch is a network transmitting chip which comprises the MDIO management interface. The CPLD is connected with the MPU through the parallel bus. The parallel bus comprises a set of control buses, a set of address buses and a set of data buses. The CPLD is connected with the switch through the MDIO management interface. According to the device expanding the MDIO interface through the parallel bus, the MPU without the MDIO interface can manage the services of the external switch and monitor the state of the external switch as well.