Moog Crossbow GNAV540 - Manual (Page 114)

Page 114 GNAV540 User Manual
7430‐0808‐01 Rev. B
Appendix B. Sample Packet—Parser Code
Overview
This section includes an example of code written in ANSI C for parsing packets from data sent by the GNAV540
Inertial Systems. This example is for reading data directly from the GNAV540 unit or from a log file.
Sample Code
The sample code contains the actual parser as well as several support functions for CRC calculation and circular
queue access.
Table 99 Code Functions
Function Description
process_xbow_packet Parse out packets from a qu
XBOW_PACKET (see below eue. Returns these fields in structure
). Checks for CRC errors
calcCRC Calculate CRC on packets.
Initialize Initialize the queue
AddQueue Add item in front of queue
DeleteQueue Return an item from the queue
peekWord Retrieve 2‐bytes from the queue, without popping
peekByte Retrieve a byte from the queue without popping
Pop Discard item(s) from queue
Size Return number of items in queue
Empty Return 1 if queue is empty, 0 if not
Full Return 1 if full, 0 if not full
The parser will parse the queue looking for packets. Once a packet is found and the CRC checks out, the packet’s
fields are placed in the XBOW_PACKET structure. The parser will then return to the caller. When no packets are
found the parser returns the value 0 to the caller.
The XBOW_PACKET structure is defined as follows:
typedef struct xbow_packet
{
unsigned short packet_type;
char length;
unsigned short crc;
char data(256);
} XBOW_PACKET;
Typically, the parser would be called within a loop in a separate process, or in some time triggered environment,
reading the queue looking for packets. A separate process might add data to this queue when it arrives. It is up to the