Compaq 850 manual Endp

Technical Reference Guide

The USB transmissions consist of packets using one of four types of formats (Figure 5-8) that include two or more of seven field types.

♦Sync Field – 8-bit field that starts every packet and is used by the receiver to align the incoming signal with the local clock.

♦Packet Identifier (PID) Field – 8-bit field sent with every packet to identify the attributes (in. out, start-of-frame (SOF), setup, data, acknowledge, stall, preamble) and the degree of error correction to be applied.

♦Address Field – 7-bit field that provides source information required in token packets.

♦Endpoint Field – 4-bit field that provides destination information required in token packets.

♦Frame Field – 11-bit field sent in Start-of-Frame (SOF) packets that are incremented by the host and sent only at the start of each frame.

♦Data Field – 0-1023-byte field of data.

♦Cyclic Redundancy Check (CRC) Field – 5- or 16-bit field used to check transmission integrity.

Token Packet

SOF Packet

Data Packet

Handshake Packet

Sync Field	PID Field	Addr.	ENDP.			CRC Field
(8 bits)	(8 bits)	Field	Field				(5 bits)
		(7 bit )	(4 bit )
Sync Field	PID Field	Frame Field		CRC Field
(8 bits)	(8 bits)	(11 bits)			(5 bits)
Sync Field	PID Field	Data Field						CRC Field
(8 bits)	(8 bits)	(0-1023 bytes)						(16 bits)
Sync Field	PID Field
(8 bits)	(8 bits)

Figure 5-7.USB Packet Formats

Data is transferred LSb first. A cyclic redundancy check (CRC) is applied to all packets (except a handshake packet). A packet causing a CRC error is generally completely ignored by the receiver.

Compaq Deskpro EXS and Workstation 300 Personal Computers 5-23

Featuring the Intel Pentium 4 Processor

First Edition - December 2000