Abstract: An asynchronous communications device includes a dual port memory, means for inputting data into the dual port memory, and means for outputting data from the dual port memory, the device being configured to operate in a receive mode or a transmit mode according to its connection in a communications system. Suitably the device includes a built-in test facility for both the transmit mode and the receive mode. Preferably a clock frequency of 80 MHz is used for the operation of the device, and a 40 MHz clock signal is used to derive the 80 MHz clock frequency, the 80 MHz frequency being formed by four 20 MHz clock frequencies arranged with their rising edges 90.degree. out of phase. There is also provided a method of producing a clock frequency by utilizing four clock pulses with their rising edges 90.degree. apart.