IMU381 Series User’s Manual________________________________________________________________________Doc# 7430-3881-02 Page 34Reads default non-volatile fieldsG = getGets current volatile fields or settingsW = writeWrites default non-volatile fields. These fields are stored in non-volatile memory anddetermine the unit’s behavior on power up. Modifying default fields take effect on thenext power up and thereafter.S = setSets current volatile fields or settings. Modifying current fields will take effectimmediately by modifying internal RAM and are lost on a power cycle.5.3.3 Payload LengthThe payload length is always a one byte unsigned character with a range of 0-255. The payloadlength byte is the length (in bytes) of the portion of the packetONLY, and does not include the CRC.5.3.4 PayloadThe payload is of variable length based on the packet type.5.3.5 16-bit CRC-CCITTPackets end with a 16-bit CRC-CCITT calculated on the entire packet excluding the 0x5555header and the CRC field itself. A discussion of the 16-bit CRC-CCITT and sample code forimplementing the computation of the CRC is included at the end of this document. This 16-bitCRC standard is maintained by the International Telecommunication Union (ITU). The highlightsare:Width = 16 bitsPolynomial 0x1021Initial value = 0xFFFFNo XOR performed on the final value.See Appendix B for sample code that implements the 16-bit CRC algorithm.5.3.6 Messaging OverviewTable 22 summarizes the messages available by IMU381 Series model. Packet types are assignedmostly using the ASCII mnemonics defined above and are indicated in the summary table belowand in the detailed sections for each command. The payload byte-length is often related to otherdata elements in the packet as defined in the table below. The referenced variables are defined inthe detailed sections following. Output messages are sent from the IMU381 Series inertial systemto the user system as a result of a poll request or a continuous packet output setting. Inputmessages are sent from the user system to the IMU381 Series inertial system and will result in anassociated 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.
