Hi Nicolaas

Welcome to the forum. A presentation would be helpful to get acquainted, just your choice.
Here a Märklin and Faller big fan from nearby Paris with a permanent, PC-operated layout (Rocrail)

You don't need an M84 since the 120171 already includes a decoder (MM2/DCC)
Here is the users manual, look at page 26 https://www.faller.de/xs_db/DOKU...tungen/120171_anl_01.pdf
The included unit understands Motorola and DCC.

It has 2 modes
- conventional one entry (START) when connected to ground, closes the gates and closing sequence, while disconnecting it starts the open gate sequence
- digital, it behaves like a switch (point).

A special entry "START" allows to program the digital address using Motorola (MM2) by simple activation of the selected switch address (from your CS2/CS3/MS2) just after connecting temporarily START to ground.

If you want absolutely to use an M84 then in conventional mode, you use one of the M84 relays to connect to ground the 120171 decoder START entry.
In this case, the decoder is powered by normal 12VAC.
Cheers
Jean

My understanding is , for this digital operation
- you power the decoder from a digital central station and prepare the new switch address (e.g. 123) you want for the gate crossing
- you connect the start entry briefly to ground and within a few seconds you send the switch control (address 123) and the decoder memorizes its new address.

Then my guess is you leave the START entry alone (open) and drive through the Red and Green commands of switch 123 (this example)
Here is my Faller level crossing (much less sophisticated) but modified to accept Faller Car System

Cheers
Jean