DMU380SA Series
User’s Manual
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Doc# 7430-0026 Rev. 01
Page 48
Sets current volatile fields or settings. Modifying current fields will take effect
immediately by modifying internal RAM and are lost on a power cycle.
6.3.3
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.
6.3.4
Payload
The payload is of variable length based on the packet type.
6.3.5
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
Telecommunication Union (ITU). The highlights are:
Width = 16 bits
Polynomial 0x1021
Initial value = 0xFFFF
No XOR performed on the final value.
See Appendix C for sample code that implements the 16-bit CRC algorithm.
6.3.6
Messaging Overview
Table 21 summarizes the messages available by DMU380SA Series model. 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 detailed sections following. Output messages
are sent from the DMU380SA Series 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 DMU380SA Series inertial system and will result in an associated Reply
Message or NAK message. Note that reply messages typically have the same
<2-byte
packet type (U2)>
as the input message that evoked it but with a different payload.
Table 21.
Message Table
ASCII
Mnemonic
<2-byte
packet
type (U2)>
<payload
byte-length
(U1)>
Description
Type
Products
Available
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
Commands
GP
0x4750
2
Get Packet
Request
Input
Message
ALL
