How many pins on a DCC decoder?

[Old Kent Biker]

I've not had much to do with DCC up to now, but I'm very interested in a new TT120 layout employing HM7000 for loco control. Can anybody explain (or point me towards) the different pin-types for DCC decoders? I've seen 4, 6, 8, NXT18 and 21-pin options. What do the different pin-outs allow control of?

[Fishmanoz]

Hi Martyn, it would take a very long reply to cover all of these connector types in full. One way to get that coverage is by going to Brian Lambert’s DCC page 1 for a start: https://www.brian-lambert.co.uk/DCC-Page-1.html

He doesn’t cover 4-pin or Next18 but note just above his pin diagrams, there is a link to the NMRA specifications for connector configurations, and they can be found there.

To give you a short summary:

• 4-pin - track input and motor control only
• 6-pin - 4-pin + directional head and tail lights (2 functions)
• 8-pin - 6-pin plus common positive plus 2 more functions for other things like cab lights, fire box etc (some have a 9th flying lead for another function
• Next18 - 8-pin plus 3 more functions. Note this is a very different miniature connect compared to the ones above
• 21MTC - 8-pin plus more functions plus special effect functions plus speaker wires

Trust that helps.

[96RAF]

Just to add to Fishy’s list.

TT:120 will only use the Next18 sound or non-sound decoders in most locos. One known exception is the Class 08 which is too small to get it in so that has a direct plug six pin decoder.

Also the N18 and 21 pin decoders support direct connection to a speaker through the pins to the loco motor board although not every loco will have a speaker pre-installed which is where the speaker socket on the decoder comes in.

Some of the extra functions spoken about are low level logic functions (3v) to control special loco features. Basic functions are high level i.e. full 12-14v output.

There is no HM7000 decoder in a four pin variant.

[Fishmanoz]

Speaker connections are an option in Next18S but you give up 2 function outputs.

Is that what HM7000TXS is using Rob.

[96RAF]

According to the spec for Next 18 Fishy the sound and non-sound decoders must be swappable and it is down to the loco manufacturer to ensure the decoder can only be fitted one way and that the loco PCB is designed so that if the wrong one is plugged in no harm derives.

From that I presume if a non-sound decoder is plugged into a sound prepped loco some precaution against damage has been designed in. And vice-versa feeding a sound output into a function path.

Outwith my bailiwick in detail. I would have to review the decoder manual.

[Fishmanoz]

Rob, I read the NMRA spec for Next18 before replying and I’ve interpreted the requirement differently to you. My assumption didn’t include whether it was a sound decoder plugged in or not, just whether you wanted to connect the speaker via those pins if sound and not have the extra functions. You could have separate flying lead connections to the speaker not via the connector. Thinking about it again though, that’s most unlikely.

I interpreted the S to be speaker, not sound, but you are clearly correct on reflection.

Given both sound and non-sound HM7000 N18 decoders, they will have to use the S connector and have no connections to those pins in the non-sound version of the decoder.