An end station port (2-6) for connection to a token passing communication system which connects a number of end station ports and in which the communications protocol under which the system operates permits more than one data frame to be transmitted by an end station within a predetermined token holding time comprises a frame assembly unit (19) for preparing a data frame, having a length not exceeding a predetermined maximum size, for transmission; and a control system (20) for controlling the passage of data to and from the end station port for transmission on to and receipt from the communication system in use. The control system (20) transmits a frame prepared by the frame assembly unit (19) when the token is held by the end station port. The control system (20) includes a monitor (21) for determining whether, following transmission of a current frame, there is sufficient time remaining to transmit another frame of the predetermined maximum size before the predetermined token holding time has expired.
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