Philips BDS4621-27 - Manual (Page 50)

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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User Manual BDS4621
BDS4621_EN.qxd 13-12-2004 10:38 Pagina 50