Understanding DCC. What is it and how it works etc

[Brian]

DCC What does it mean and how does it work?
DC = Direct Current also called Analogue
DCC = Digital Command Control or also called Digital.

What is DCC?
DCC allows more than one loco to be on the same piece of track and only the one selected will move or operate a device on the loco - lights or sounds, the others remain doing whatever it was last instructed to do – Stop, move forwards at half speed etc. How is this possible? Each loco has what is called a mobile Decoder fitted. This is “tuned” to receive only instructions sent to it. The “Tuning” is called its Address number and can be in the range of 1 to 9999. All new decoders are supplied set to a default address number of 03 and ideally once proved to be working correctly, is to give it a new address number. This range of address number will be console dependent. Some very basic systems only allow a few address numbers, other systems several, while up market systems allow the full 1 to 9999 range to be used. The address number is stored in the locos decoder and therefore a loco can be moved from one DCC system to another and still retain the address number allocated. Of course, if moving to a lower or more basic system and a higher address number has been set that is outside the range of the basic system it will not work, as that system cannot send the higher address number and therefore the decoder would then have to be readdressed to a suitable number to match the system being used.

How it works.... Consider our DCC Fitted loco as you in a house in your road living at number 46 (46 is the locos address number) and the postman is the DCC system command carrier. Currently you at No 46 have no new instructions, so you remain doing whatever you were last told to do. When a commend is issued (a letter addressed to No 46 in this scenario) the postman carries this around with him (data on the DCC rails) all people in their houses can see the Postman, but they do not receive the letter as they are not at No 46. The letter is delivered to your number 46 house (Data flows to all decoders, but only the one with the correct address number receives the data). Once you open the letter the instructions inside it tell you and only you to do something like move forward at a quarter speed (You then do this until told later to do something else) So we have sent a digital command to the loco. This is an overly simplistic understanding of how all the decoders all listen for instructions, but only the one with the unique address number responds to the commands being sent.

There are basically three types on loco available
1) DC (not DCC Ready) usually locos sold in the past and they do not have any plug-in decoder socket fitted. The loco is designed to run on DC power as sold. Some if not the vast majority can with care be converted to DCC.
2) DCC Ready, Modern locos that are supplied under this heading have a factory fitted socket that as sold has a DC operation plug fitted and only allows the loco to work as sold only on DC. But by removing the plug and fitting a suitable decoder of the correct pin arrangement the loco will become DCC Fitted and works then on both DCC and DC
3) DCC Fitted. As sold new the loco has a factory fitted (or dealer fitted) decoder installed and is immediately ready to run on DCC powered systems. Normally the loco will also run on DC power too. But may not perform as well on DC as it does on DCC.

Can I run a DC loco on DCC?
No, If a DC loco is left on DCC powered rails its very likely to suffer a burnt out motor. Some DCC systems offer a facility to run one DC loco on address number 0 (zero) but this is not a recommended practice and ideally the loco should have a decoder fitted ASAP or not be placed on DCC powered rails.

Can I run a DCC loco on DC?
Usually Yes. The exception is where CV29 has been adjusted to prevent DC operation. Some decoders / loco combinations may not perform as well on DC as they do on DCC. Not all Sounds will operate on DC, only a basic running sound is the norm.

What is the basic difference between DC and DCC?
DC uses a variable DC voltage applied to the rails nominally 0v to 12v. The more volts applied the faster the loco will move. To stop the loco the rail volts are turned to 0v (zero volts).
DCC use a sort of AC power, but its not like mains AC power, so the analogy between DCC and AC is not the same. Rail volts are continuously applied to the rails at around 13 to 15 volts AC but some DCC system allow the user to set the rail voltage themselves. DCC frequency is around 7KHz to 9KHz nominally 8KHz. That is an average of 8000 times a second. Mains frequency is at 50Hz in the UK and 60Hz in America. That’s 50 or 60 times a second. Note; "old school" readers will remember this as Cycles per Second! Mains AC is a sinewave where as DCC is more of a Square‘ish wave with data superimposed on this.
You can read DCC volts with an multimeter set to its AC volts range. But many domestic meters will not be able to cope with the high frequency and therefore it may well give an inaccurate reading. But whatever is read at the feed should be virtually the same everywhere around the layout.

Will the points on my DC layout work if I convert to DCC?
Yes, there is no such thing as a DC or DCC point. If it works on DC then it will work on DCC. There is a small catch though! On DC its common to use the point to turn off the rail power to the unset direction thereby allowing a loco to sit in a siding with the point set against it and the loco doesn’t move. On DCC it is regarded as normal to have all tracks live regardless of the points position. This then allows loco sound to operate, loco lights to be turned on or off where fitted or movements made within that siding or area even with the main entrance point set against that direction. To overcome the self isolation points produce there are a couple of options.
a) If using Hornby points fit their Electro Point Clips – Two clips per point.
b) Or better is to instal new rail feeds to the two rails after the Vee rail ends of the point and ensure these feeds are of the correct polarity.

Will my existing point motors work on DCC? No, not if you require DCC operation of them. They must have an interface between the DCC system and the motor (or other accessories) these interface devices are called Accessory Decoders or sometimes are known as Stationary Decoders. Then the DCC is feed to the Accessory decoder, this converts the Digital instructions received and outputs power to the appropriate device connected to its output(s).

Can I mix Analogue point motor control with DCC loco control?
Yes, you can operate all accessories totally separately from the DCC, but you must ensure the two never connect together. So as an example, all point motors could be operated by Analogue – passing contact levers or suitable switches etc and feed from a totally separate power source and wired separately from the DCC system.

How many locos can I have on my DCC powered layout?
As many as you like so long as each loco has a decoder fitted. Currently the maximum DCC address number is 9999. How many you can control at once is a different thing, I find three a strain and four virtually impossible! But locos stabled or sitting stationary on DCC powered rails draw very little power in their standby mode. Probably less than 10 milliamps each. Add lights being On and that power draw increases to around 20milliamp. Turn sounds On and it can rise to 100 to 150milliamps per loco. These are not accurate figures and are just rule of thumb idea and much will be determined by the decoder and the loco features. Note that no two identical locos will consume the same identical power!

Can I mix DCC and DC tracks on my layout?
Yes, but do so with great caution. The two systems must be kept totally separate. On no account allow DC to connect to DCC, if they do one or both systems will become terminal! Never allow a cross over set of points to exist between two loops or separate lines where one loop or line is feed with DC and the other with DCC. A loco traversing the point will connect loop or line A to loop or line B. Result = “Poof” your DCC system will probably stop working!

How much will it cost me to convert to DCC?
Very hard to say but as a minimum work on a ballpark basic new DCC system as being around £80 to £150. More if an up market DCC console is purchased. Loco decoders from around £12 up, with mid range versions selling for about £20-£25 each.
You do not have to convert all your locos to DCC immediately. Just convert a few initially, then convert others as time and finances allow. However, keep all unconverted locos off the DCC powered rails.

Should I install a DCC Bus pair of wires?
You do not have to do this, but then you are relying on the metal rail joiners (Fishplates) passing power and data rail to rail. Metal fishplates are the weak link in any model railway where power transfer is involved. Even on DC layouts they can easily pose a problem! The very best option for power and data transfer is to install a DCC power Bus pair of wires, then tap off this pair to feed the rails at as many places as possible or practicable. I for example ensure for total reliability, that every piece of track has a dropper pair fitted, but some may like to add droppers to every other tracks section and rely on those metal Fishplates for the connect rail to rail too. Installing a DCC Bus pair is not suitable if you continually change your track plan. Here you will rely normally on those pesky Fishplates! But once a track plan has been settled upon and the track is to be permanently fixed down, a DCC Bus pair will ensure far better power and data transfer around the whole layout.

What size of wire to use?
For the actual power Bus pair use as a minimum ideally 32/0.2mm equipment wire or other similar sized flexible wire. However, if your layout is permanent then solid core wire can be used, such as the wire stripped from the outer insulation of mains cable. Here 1.5mm² is about the smallest I would recommend. Remember, you cannot go too large in Bus wire size but too small may lead to issues!
Droppers from the Bus pair to the rails can be 7/0.2mm if kept short and to no greater than 350mm in length and where all sections of track are fitted with droppers. Or use 16/0.2mm equipment wire for longer dropper wire runs or where every section of track is not feed.