Moog Crossbow GNAV540 - Manual (Page 63)

540 User
0808‐01 Rev. B
Page 63 GNAV
7430‐ Manual
Payload Length
The payload length is always a one byte unsigned character with a range of 0‐255. The payload length byte is the
length (in bytes) of the <variable length payload> portion of the packet ONLY, and does not include the CRC.
Payload
The payload is of variable length based on the packet type.
16‐Bit CRC‐CCITT
Packets end with a 16‐bit CRC‐CCITT calculated on the entire packet excluding the 0x5555 header and the CRC field
itself. A discussion of the 16‐bit CRC‐CCITT and sample code for implementing the computation of the CRC is
included at the end of this document. This 16‐bit CRC standard is maintained by the International
Union (ITU). Telecommunication
Width: 16 bits
Polynomial: 0x1021
Initial value: 0xFFFF
OR is performed on the final value. No X
Refer to Appendix B. Sample Packet—Parser Code for sample code that implements the 16‐bit CRC algorithm.
Messaging Overview
The following table summarizes the messages available with the GNAV540 unit. Packet types are assigned mostly
using the ASCII mnemonics defined above and are indicated in the summary table below and in the detailed sections
for each command. The payload byte‐length is often related to other data elements in the packet as defined in the
table below. The referenced variables are defined in the following sections.
Output messages are sent from the GNAV540 inertial system to the user system as a result of a poll request or a
continuous packet output setting. Input messages are sent from the user system to the GNAV540 inertial system
and will result in an associated Reply Message or NAK message. Reply messages typically have the same <2­byte
acket type (U2)> p
as the input message that evoked it but with a different payload.
Table 14 Message Table
ASCII
Mnemonic <2‐byte packet
ype (U2)> t
<payload
byte‐length
U1)> (
Description Type
Available
unctions F
Link Test
PK 0x504B 0 Ping Command
and Response
Input/Reply Message ALL
CH 0x4348 N Echo Command
and Response
Input/Reply Message ALL
Interactive Co mmands
GP 0x4750 2 Get Packet
Request Input Message ALL
AR 0x4152 0 Algorithm Reset Input/Reply Message VG,AHRS, NAV
SR 0x5352 0 Software Reset Input/Reply Message ALL
NAK 0x1515 2 Error Response Reply Message ALL