This format, which is a envelope format for an envelope-and-payload style of data, describes data as bigEndian, but LSBF. https://www.sisostds.org/DesktopModules/Bring2mind/DMX/Download.aspx?Command=Core_Download&EntryId=43048&PortalId=0&TabId=105 Thanks to Marshall Miller of Quark Security for this insight. This extracted from the dev@daffodil.apache.org mailing list. One can workaround this problem in Daffodil by way of dfdl:inputValueCalc/dfdl:outputValueCalc and hidden groups. But that is, perhaps, unsatisfying. Example: Data is comprised of 3 unsigned integer bitfields: slot1 is 12 bits slot2 is 32 bits slot3 is 20 bits Given an input with the following bytes in hex: 01 23 45 67 89 AB CD EF Parsed results need to be (in hex): slot1: 0567 slot2: DEF01234 slot3: 089ABC Explanation of results: Since the bit fields need to consume data in a leastSignificantBitFirst order, it is easier to represent the input in right-to-left order where bits are consumed starting at the right and moving toward the left. Since words are 4 bytes, and in big endian order, the lowest bit in the 4th byte is the first bit to be consumed. Reordering the input data and consuming data from right to left yields the correct field boundaries. 89 AB CD EF 01 23 45 67 parse fields right to left 05 06 07 08 01 02 03 04 input byte position slot1 consumes the right 3 nibbles, slot2 consumes the next 8 nibbles, and slot3 consumes the left 5 nibbles. Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy <http://www.ogf.org/About/abt_policies.php>