VIA Technologies, Inc.

Preliminary VT6304

9

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.