Philips BDS4621/27 User Manual

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1.2. Send Command

1.1.2. Introduction

Command packets consist of "Header" and

"Payload".The Packet Header is consistent for all

packets.The Packet Payload type and content

varies based on the type of packet sent.The

entire packet size is variable, being the sum of

the fixed-size Packet Header and variable-sized

Packet Payload.

Packet Header (fixed size) Packet Payload (variable size)

Tabl e 4

Packet Header Format

Introduction

All Packets use the same Packet Header format

illustrated in Table 5.

Byte 0 1 2 3 4 5 6

Magic Number Type Packet Payload Size CRC

0xBE 0xEF type size_lo size_hi crc_lo crc_hi

Tabl e 5

The Packet Header size is fixed at seven bytes

(Intel byte ordering is used).The following code

fragments are taken from these source files

The Packet Header definition is shown below:

typedef struct

{

BYTE ePacketType;//type of the

payload

WORD nPacketSize;//size of the

payload

WORD nCRCPacket;//CRC for the

entire packet

} PACKET_HEADER;

Magic Number

The Magic Number is a fixed value that is used

to insure packet alignment if there are partial

packets received or bytes lost.

The Magic Number is a WORD in length (2

bytes).The Magic Number value is 0xEFBE.

Because Intel byte ordering is used, the ls-byte

of the word is sent first (byte0 = 0xBE), then the

ms-byte (byte1 = 0xEF).

User Manual BDS4621

BDS4621_EN.qxd 13-12-2004 10:38 Pagina 50

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