Decoders - one or two & which one?

[JohanC]

Reading through DCC posts, some refer to locos requiring TWO decoders. Why is this?

I have always used basic Hornby 8 pin decoders. What is the advantage of other types? 21 pin won't fit and my locos don't have speakers.

Any thoughts would be appreciated.

John.

[Fishmanoz]

It would be quite unusual to need 2 decoders in a loco. The only reason I can think of is if you had more features to control than the 4 that can be handled by a standard 4 function 8 pin type. However, a modern double ended train with a power car at one end and a dummy car at the other is likely to need a decoder in each, the one in the dummy car to run directional lighting. 

[Rog RJ]

There are some models that need more than two decoders.  The Ba**mann class 108 3 car DMU is one such example, which needs three decoders.  If the the decoder only has to control the lighting functions you can use special function only decoders, although they may cost more than a standard, basic decoder but do have certain features that may make them a better choice for some situations.

Different decoders have different characteristics, power handling capability and functions.  The basic Hornby decoder has minimal adjustment possibilities via the CV settings and only 0.5 Amp power handling whilst the Hornby Sapphire has many more adjustments available, a higher power capacity of 1 Amp and extra features such as a fuel load which can be set to reduce as the model runs.  A bit of a gimmick IMHO but may add interest for some users.  Some other makes of decoders may have many more function outputs on them for controlling lights, horns, smoke generators and uncouplers etc.

The higher number of function outputs and sound lead to the requirement for decoders to have more connections available and so the 21 pin decoder was introduced as an interim measure while a new standard was developed.  This is now available and is known as a Plux connector.  It has 22 pins but one of those pin positions is used as an index position, so it is effectively still a 21 pin connector.  The interesting thing about the Plux standard is that the larger sockets can have Plux decoders with fewer pins plugged in.

There are, I believe, 4 sizes of Plux connectors, 8, 12, 16 and 22 pin. Any smaller size of decoder can be plugged into the same or larger size of socket, though you may not be able to use some of the functions connected to the larger socket.  The plux standard is relatively new and not many manufacturers are using yet but there are a few.

There will be some models (many?) which will have a smaller size of socket fitted instead of the the full 22 pin one.

[JohanC]

Thanks for the comprehensive replies, very interesting. John

[Graskie]

Johan, I only run sound locos, steam and diesel, nowadays. More recently I have bought several with two decoders.

I have a 2 car DMU and, as WTD said, that operates not only the directional but carriage lights entirely independant from the power car which has been refitted with a more versatile sound and lights decoder of its own. The two DMU units are not attached electrically at all, but operate separately under the same ID.

I also have some more recent steam locos with separate decoders for smoke units, which produces much more smoke than a combined one. For these, separate IDs are needed, to avoid running the smoke unit with no smoke oil in it, which would burn it out. 

[walkingthedog]

'I have a 2 car DMU and, as WTD said'

I didn't say it honestly Graskie. 

[Graskie]

Sorry, WTD. Instead of "that" in my 2nd paragraph, read "the dummy car not only can but does." Sounds appropriate, doesn't it?