Page 15 - January February 2017
P. 15

 In the first part of this article we discussed what DCC is, what the components are, and how to choose and buy a DCC system. In this part, we’re going to take a look in more detail at Locomotive decoders and at the DCC layout.
Locomotive decoders
Mobile decoders (as opposed to accessory decoders) are what go in locomotive. They run the motor, and operate the lights. Sound decoders do all that and make the sound happen, too. There's a bewildering choice these days of decoders, what they do, and how they're installed, so let's try and make a little sense of it all.
First issue with choosing a decoder is how it gets installed into the locomotive. Shortcut all of that by exploring the major decoder manufacturers web sites - Digitrax, Soundtraxx, TCS, ESU - and others; look at their decoder selectors. I'd encourage you to check these out; they will show you what decoder is suitable. There are often installation guides available, too. Using these charts and guides will shortcut many of the choices that you might otherwise have to make. Of course, for installs in brass and in older locos, you'll have to make these choices yourself.
As to the installation, there are three main choices. First is the direct plug-in style, which is how all recent locomotives are sold - usually with the DCC ready description on the packaging. Such locos have one of three main plug-socket styles. Commonest in HO is the NMRA 8-pin design. Most decoders with 8-pin plugs have the plug - two rows of 4 pins - at the end of some short wires, which will be in the NMRA standard colours too. The loco will come with a blanking plug installed in the socket to operate the loco on DC. To install the decoder, you merely pull that blanking plug out, and plug the decoder in. The circuit board should identify Pin 1 of the socket somehow; a number 1, and asterisk or a small arrowhead symbol are common identifiers here. Just plug the pin with the orange wire connected to it to Pin 1, and you're done. If you do plug it in the wrong way, the loco will run backwards and the lights won't work, but no harm will be done. Do beware, though: some 8-pin decoders have the pins on the bottom of the decoder board itself, and this may have issues in actually fitting the loco body back on with the decoder in place.
Soundtraxx Econami Decoder with an 8-pin plug installed in a Bachmann trolley car. Photo: Mick Moignard
Common in N-scale is a 6-pin socket. Again, you'll find that pin 1 is called out on both socket and decoder, and again, its a matter of removing a blanking plug and plugging in the decoder, being really careful to ensure that you get it the right way round.
Increasingly common these days is a 21-pin arrangement - two rows of 11 pins with one missing. This time the decoder carries the socket part. 21 pins means that it can carry more functions - more lights, and not uncommonly, speaker connections for sound. And again, they're straight plugins after removing a blanking socket, making sure that the pins are aligned properly with the blank location.
 21-pin and 8-pin decoders shown with a 10p coin for size comaparisons. Photo: Mick Moignard
Next set of options is common for HO and N-scale diesel and electric models, and this is what's called a board-replacement decoder. These are sold for almost every RTR North American style diesel or electric model, and, as the name suggests, are intended to be a direct replacement for the DC circuit board already installed inside the loco. These tend to be very easy to fit, requiring just that the wires clipped to the original board are removed and then replaced on the new board, like for like. Personally I like to solder the wires to the decoder rather than relying on those little plastic retainers. However, older locos of the Athearn Blue-box generation don't have factory circuit boards, and will need to be hardwired.
  N-scale Kato SD70Ace with Digitrax SDN144K1E board replacement decoder. Photo: Mick Moignard
So, last and ultimate choice is the hardwired decoder. This is one that comes with a set of bare wires. These wires have standardised colours, which you'll quickly learn. Red and black carry track power to the decoder, red being connected usually to the right rail when the loco is going forwards. Orange and grey connect the motor to the decoder, orange being connected to the motor terminal that was connected in the DC wiring to the right - red – rail. It is important to ensure that























































































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