Abstract: An adapter for transferring blocks of data of a variable size to at least one destination adapter, each of the adapters being plugged in a respective slot and connected to a main system bus which is controlled by a central backplane card. In each adapter, the transmission of one block of data is provided, in part, by a device, operative in response to the detection of an ACK acknowledge word giving access to the bus, for generating a signalling word to be transmitted to a particular destination adapter which will actually receive the transmitted data; the signalling word comprising information characterizing the type of command which the transmitting adapter sends as a request to the backplane card, the address of the particular adapter which will receive the transmitted data, the size of the block of the transmitted data and the address of the transmitting adapter. The adapter is best suited for use in multimedia systems where variable size data blocks are required to transmit data, voice, video, etc.