DCC chip differences

Discussion in 'Techniques' started by Barnaby, 10 November 2017.

  1. Barnaby

    Barnaby Western Thunderer


    I notice some new 7mm locos are showing they are DCC ready but while both are by very good Manufacturers one says 8Pin while the other says 21Pin ready.
    So my question is what is the difference between them, is it just that the 21Pin will have more options to control or is there something else?

  2. simond

    simond Western Thunderer

    Hi Barnaby

    8-pin is a basic plug in standard, needs 4 pins for pickups and motor, I would have to check but the others are lighting iirc. Normally the decoder has a short harness with a plug (male) on the end, and the loco has a socket. You can wire other stuff to the decoder eg speaker if it has sound, KA capacitor, etc, but they are not through the plug. You can also get a blanking plug to convert back to analog dc.

    21-pin includes more lights and the speaker and the KA connections. The decoder has a 22-pin socket (with a blank) and the loco has the 21-pin plug. I guess there’s an analog blanking plug too, but I’ve not seen one.

    If you’re doing your own install, you can buy the mating connectors from the usual dealers, although I think the 21-pin plug PCB is an ESU item.

    Barnaby and 3 LINK like this.
  3. SoundsLoco

    SoundsLoco Member

    In essence Simon is correct, the 21 pin interface is designed to avoid needing to add wires to the decoder to access higher functions.

    But in practice, it's best to think of what will be available from a 21 pin interface as being 'up to' 21 individual connections.

    The decoder will have it's own number of 'connected' pins and the loco PCB will have another, often smaller, number of connections to the 21 pin positions in the interface. Until very recently the PCBs of UK models normally only had outlets for 4 Function Outputs, reflecting the limited features common with UK models.

    Some more adventurous manufacturers are now exploring higher numbers of functions for more interesting lighting features.

    This has revealed a hidden problem - there are or were 2 wiring standards for 21 pin decoders. the divergance occurs from Aux 3 and above. Some decdoer follow the 'all function outputs are open collector types' (often called full power) whilst ESU and latterly TCS, who have changed from one to the other, follow the 'AUX 3 and above Logic Level only'.

    You may recall the discussions about this on the Dapol 08.

    The non-wired decoders are cheaper and fitting them on the production line is more efficient so a decision to use this type may be more about reducing costs than actually providing additional functionality.

    Kind regards,

    daifly and Barnaby like this.
  4. Barnaby

    Barnaby Western Thunderer

    Thanks for your fulsome explanations Simon & Paul.