Is there anybody who could tell whether the mfx decoders show 14 or 27 speed steps when controlled by the Märklin/Motorola format?

The Märklin/Motorola format defines only 14 speed steps. However, at least the 6090x-series and non-mfx C-sine decoders show 27 different speed steps in each direction (not counting total stop as a speed step). The way this is controlled is that the decoder goes through odd speed steps upwards, and even speed steps downwards. So a speed step command sequence 0-1-2-3-4-3-2-1-0 will result in the loco speed steps 0-1-3-5-7-6-4-2-0. It is difficult to take advantage of this when running manually from the 6021 speed knob, but with computer control the extra speed steps can be controlled as you like, by sending the speed commands in pairs if necessary.

Now, the question is whether the new mfx decoders show the same behaviour when controlled by the Märklin/Motorola protocol, or if they only show 14 speed steps. I am not able to test myself since I do not have any mfx loco yet.

A side note here is that the ESU LokPilot offers 28 speed steps with a slightly modified version of the Märklin/Motorola protocol. The 14 extra speed steps are not reachable via a 6021, but I think an Intellibox can use them.

Do you mean that you only see 14 speed steps with mfx locos? Or do you mean you don't see the extra speed steps that are evidently there in the 6090x locos?

OK, thanks h-zero.

So, the mfx decoders actually give fewer speed steps than the old 6090x, when controlled with the Märklin/Motorola format. This means that I will stick to 6090x when converting old locos, at least for the moment.

Yes, I have observed that too.