a communications interface device has circuitry for receiving digital signals in the form of characters according to a protocol defining that a communications link is initialized by a series of start characters , each having a specified duration . the receiving circuitry can assume a first state in which characters can be detected , and a second state in which detection of characters is not possible . power consumption is higher in the first state than in the second state . the device can alternate the receiving circuitry periodically between the first state and the second state with a period shorter than the duration of the series of start characters . in each period the receiving circuitry assumes the first state for a time longer than the duration of a start character . a communications link is established if a character is detected . the power consumption can be reduced considerably without causing unnecessary inconvenience and time consumption for the user .