VT6304

The VT6304 provides three-port physical layer function in a cable-based IEEE 1394-1995 network. Each cable port incorporates two differential line transceivers. The transceivers include circuitry to monitor the line conditions as needed for determining connection status, for initialization and arbitration, and for packet reception and transmission.

Data bits to be transmitted through the cable ports are received from the Link on 2/4/8 data lines (D[0:7]), and are latched internally in the VT6304 in synchronization with the 49.152-MHZ system clock. These bits are combined serially, encoded, and transmitted at 98.304 , 196.608 or 393.216 Mbits/S as the outbound data-strobe information stream. During transmission, the encoded data transmitted differential on the TPB cable pair(s), and the encoded strobe information is transmitted differentially on the TPA cable pair(s). During packet reception the TPA and TPB transmitters of the receiving cable port are disabled, and the receivers for that port are enabled. The encoded data information is received on the TPA cable pair, and the encoded Strobe information is received on the TPB cable pair. The received data-strobe information is decoded to recover the receive clock signal and the serial data bits. The serial data bits are split into two or four parallel transmitted (repeated) out of the other active (connected) cable ports.

Both the TPA and TPB cable interfaces incorporate differential comparators to monitor the line states during initialization and arbitration. The output of these comparators are used by the internal logic to determine the arbitration status. The TPA channel monitors the incoming cable common-mode voltage. The value of this common mode voltage is used during arbitration to set the speed of the next packet transmission. In addition, the TPB channel monitors the incoming cable common-mode voltage for the presence of the remotely supplied twisted-pair bias voltage. The presence or absence of this common-mode voltage is used as an indication of cable connection status. The cable connection status signal is internally debounced in the VT6304 on a cable disconnect-to-connect. The debounced cable connection status signal initiates a bus reset. On a cable disconnect-to-connect a debounce delay is incorporated. There is no delay on a cable disconnect.