VITESSE
SEMICONDUCTOR CORPORATION
Data Sheet
High Performance Serial
Backplane Transceiver
VSC870
2.0 Packet Mode
2.1 Overview
In Packet Mode the BYPASS signal is set LOW to allow the transceiver to utilize the built-in retransmission,
camp-on, virtual output queue and delayed read enable logic. In addition, the signal CELLSYN is held LOW to
disable the cell synchronization process. A special command word can be sent through the transceiver to the switch
chip requesting connection to one or multiple output ports. Acknowledge (ACK) information will be returned to the
transceiver from the switch allowing the port card to start transmitting data (see Application Note 31: Design Guide
for a Packet Based Switch with Distributed Control). This special command word, a Connection Request (CRQ)
Command, can be put on the front of the data packet to make the packet self routing. In this mode of operation, no
controller chip needs to be connected to the switch chip as the switch chip handles all arbitration for connection
requests (see the VSC880 data sheet). If the system designer wants to bypass the features described in this section, the
signal BYPASS can be set HIGH. The user will then have direct control of the data stream between the transceiver
and the switch. A picture of a Packet Mode system is shown below. The minimum packet size in this mode of
operation is 4 words or 16 bytes.
Figure 1: Packet Mode System
Port Card
TXFIFO
Trans
Switch Card
RXFIFO
VSC870
Switch
Chip
VSC880
Port Card
TXFIFO
RXFIFO
Trans
VSC870
2.2 Data Encoding Format
The data and command words are described in section 1.0. In this section the Connection Request (CRQ) word
format at 4 interfaces (transceiver parallel transmit port, serial line to switch, serial line from switch and transceiver
parallel receive port) is described. In this mode the TXTYP[1:0] and RXTYP[1:0] bits are encoded differently than
the overhead bits (B[1:0]) to effectively support a variety of features in the Packet Mode. The encoding for these data
types are described in the following section.
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VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
