Older Tacx wireless trainers are not ANT+ FE-C compatible and use a proprietary ANT-based protocol instead.
This summary is based on work done by @totalreverse, @fluxoid-org, @mikeymclellan, @m472 and @rjohn65 (see sources) along with own research.
This document is a work in progress and represents a best guess. Some of the information has not been verified with actual hardware.
All messages are sent as broadcast data (unless otherwise noted). All integer values are big-endian (MSB first).