An adapter includes a mechanical frame, which is configured to be inserted into a four-channel Small Form-Factor Pluggable (SFP) receptacle and to receive inside the frame a single-channel SFP cable connector. First electrical terminals, held by the mechanical frame, are configured to mate with respective first pins of the receptacle. Second electrical terminals, held within the mechanical frame, are configured to mate with respective second pins of the connector. Circuitry couples the first and second electrical terminals so as to enable communication between the connector and one channel of the receptacle while terminating the remaining channels of the receptacle.