Philips BDS4621 user manual Send Command Introduction, Byte

User Manual BDS4621

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)

Table 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
			Table 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).

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