V.FC was intended to take some of the techniques being proposed for V.34 and put them into a real modem that people could use. This also gave a lot of people the opportunity to try out 28.8 kilobit per second operation for the first time. There was never any intention from Hayes or Rockwell (who worked together for two years on V.FC) that V.FC would be compatible with V.34 - even if they had wanted it, others would have made sure it didn't happen! In fact, they made the start-up deliberately different from V.34 so that it would be easy to distinguish between the two and easier to make dual-mode V.FC/V.34 modems.
V.FC is quite different from V.34. Most of the signal-processing algorithms, whilst based on the same theory, are implemented in different ways. V.34 has some extra things like a secondary channel and a special mode for 28.8 kilobit per second fax.
The Rockwell V.FC implementation uses a single-chip mask-programmed DSP for all the signal processing functions. You can also buy a modem controller chip from Rockwell to go with it which implements AT commands, error-control and compression. Hayes made their own controller using the Motorola 68302 processor. When it comes to an upgrade from V.FC to V.34 you have to have a new, masked DSP chip and new controller firmware to implement all the V.34-specific features. This means that Rockwell-DSP based modems must be returned to the manufacturer for upgrade. Upgraded modems will talk to either V.FC or V.34 modems.