VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC870

High Performance Serial

Backplane Transceiver

Page 12

G52190-0, Rev 4.1

01/05/01

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Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

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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

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.

Port Card

Trans

RXFIFO

TXFIFO

Port Card

Trans

RXFIFO

TXFIFO

Switch

Chip

Switch Card

VSC870

VSC870

VSC880