The decoders are all the same, it just depends on how many connections you need.

21 pin decoders are for those locomotives that have a 21 pin MTC connector and are a quick and easy swap in, swap out decoder.
21 pins aren't really needed though but allows for plenty of flexibility and forward compatibility.

8 pin decoders are the most common type. A lot of manufacturers make their digital locomotives with an 8 pin socket, again easy to replace a decoder.
Most Roco loks have an 8 pin interface.

6 pin decoders are used in locomotives with less space or that don't need it. I have a fleischmann lok or two that come with a 6 pin socket.

All of these can be made to work with each other by manually connecting wires. They are just 3 different standards to make changing decoders easy.

If upgrading an existing digital lok to sound for example, you would choose the connection type that the locomotive came with.


In your case, if you are upgrading an old analog locomotive you can buy any of the 3 interfaces and hard wire it to the loks outputs.

If you wanted to be able to easily swap decoders, you should also buy for example an 8 pin socket, wire that up and then just plug an 8 pin decoder in.

Rovert,

All depends which loco(s) you like to convert.
Forget about the 6 pin decoder.

8 pin plug decoder (you need an 8 pin socket wire harness) is the same as a 21 pin decoder (you need an adapterplate) ,except you only have 1 set of wires, the decoder is pushed into the adapterplate.

you have 3 types of adapterplates:
1.) ESU 51967 = Aux 1 & Aux 2
2.) ESU 51968 = Aux 1, Aux 2, Aux 3, Aux 4.
3.) a new Adapterplate but hasnt come onto the market as yet. ( 6 Aux functions) will be released later in the year.

I have done many conversions and I do it professionally for members, I would not recommend the permanent magnet with the original 3 pole motor, you don't have a slow & smooth starting mode.

regards.,

John