Specifies a physical-level, point-to-point, full-duplex, link interface for reliable, flow-controlled transmission of user data at 6 400 Mbit/s per direction, across distances of up to 1 km. Characteristics of a HIPPI-6400-PH physical-layer interface include: - user data transfer bandwidth of 6 400 Mbit/s (800 MByte/s); - a full-duplex link capable of independent full-bandwidth transfers in both directions simultaneously; - four virtual circuits providing a limited multiplexing capability; - a fixed-size transfer unit, i.e., a 32-byte micropacket, for hardware efficiency; - a small transfer unit resulting in low latency for short Messages, and a component for large transfers; - credit-based flow control that prevents buffer overflow; - end-to-end, as well as link-to-link, checksums; - automatic retransmission to correct flawed data providing guaranteed, in-order, reliable, data delivery; - an a.c. coupled parallel electrical interface for driving parallel copper cable over limited distances; - a parallel electrical interface for driving a local optical interface for longer distances.