Moog Crossbow GNAV540 - Manual (Page 61)

GNAV540 User Manual
7430‐0808‐01 Rev. B Page 61
Chapter 8. Data Packet Structure
NOTE: This section of the manual assumes the reader is familiar with ANSI C programming language and data type
conventions.
The unit supports a common packet structure that includes both command or input data packets, and measurement
output or response packet formats. This section of the manual explains these packet formats as well as the
supported commands. NAV‐VIEW 2.2 also features a number of tools that can help a user understand the packet
types available and the information contained within the packets. For an example of the code required to parse
input data packets, please see refer to Appendix B. Sample Packet—Parser Code.
General Settings
The serial port settings are RS232 with 1 start bit, 8 data bits, no parity bit, 1 stop bit, and no flow control. Standard
0, 19200, 38400, and 57600. baud rates supported are: 960
Common definitions include:
• A word is defined as 2 bytes, which are 16 bits.
• All communications to and from the unit are packets that start with a single word alternating bit preamble
0x5555. This is the ASCII string “UU”.
• All multiple byte values are transmitted Big Endian (Most Significant Byte First).
• All communication packets end with a single word CRC (2 bytes). CRC’s are calculated on all packet bytes
excluding the preamble and CRC itself. Input packets with incorrect CRC’s will be ignored.
• Each complete communication packet must be transmitted to the GNAV540 inertial system within a four (4)
second period.
Number Formats
Number Format Conventions include:
• 0x as a prefix to hexadecimal values
• Single quotes (‘’) to delimit ASCII characters
• No prefix or delimiters to specify decimal values.
The following table defines number formats:
Table 12 Number Formats
Descriptor Description Size (bytes) Comment Range
U1 Unsigned Char 1 — 0 to 255
U2 Unsigned Short 2 — 0 to 65535
U4 Unsigned Int 4 — 0 to 2^32‐1
I2 Signed Short 2 2’s Complement ‐2^15 to 2^15‐1
I2* Signed Short 2 Shifted 2’s Complement Shifted to specified range
I4 Signed Int 4 2’s Complement ‐2^31 to 2^31‐1