Thursday, September 10, 2009

A Note on Digital Command Control

Digital command control (DCC) had already been on the scene for a while when I began working on my most recent railroad, and I knew from the outset that I was going to avail myself of this newer technology. For those following my blog who aren't modelers, here's a quick and dirty primer on DCC.

Back in the day one ran trains with one or more DC power packs (rectifiers) feeding electricity to the track. This was all well and good, but the problem was that if you put two locomotives (or three or four) on the track at the same time both (all) would "respond" to the juice flowing through the rails, and move simultaneously (and if their motors' polarity happened to be different then they'd run in opposite directions simultaneously). Most inconvenient, to be sure (and shades of Gomez Addams . . .). The only way to get around this was to electrically isolate sections of track into "blocks" by having the power routed first to double-pole/double-throw (DPDT) off-on switches and from there to the track blocks. You could connect two power packs to the tracks then and by alternatively allowing power to be sent to this block or that block (by toggling the DPDT switches) it was possible to run more than one loco (thus trains) at the same time. Obviously things could get a little complex, and the wiring was a chore, to put it mildly. And if you had a large railroad the number of "blocks" could become unwieldy. Additionally, running trains was often an effort in memory and concentration, as you would have to make sure that train A (loco A) didn't accidentally drift into a block where train B (loco B) might have been under power, and so on. People back then used (bought or devised) all sorts of ingenious electrical circuitry to facilitate easier running of trains in this fashion, but the fact remains that it was tedious and cumbersome — a pain in the butt, frankly. All of my railroads from when I first began getting serious about the hobby, in the early 80s (and I built four of them in that decade — two rather large) till '05 were done using this method . . . standard DC. (Suffice it to say that nothing I'd done when I was very young involved much complex wiring at all . . .)

I won't get into the history of digital command control here, but will say that most modelers are probably grateful for its appearance on the scene. The basic idea is that instead of 12-volt DC current going to the tracks (in whatever roundabout fashion) digital information (sent in packets) is passed along simultaneously with that current. Each locomotive is equipped with a chip (a decoder) that responds only to the signals (the "address") it's set up to respond to. That's basically it. Locomotive A responds only to the signals recognized by its own decoder, loco B the same, and so on. You can have as many locomotives (thus trains, if you care to see it that way) as you wish on the tracks at the same time . . . or as many as your command unit will allow you to address. So now the power packs are actually little computers, small digital switching devices, basically, that also provide the 12-volt DC current; and you "dial in" locomotive A, or B, or whichever you choose, and move only that one around. Brilliant. Ah, technology.

Most basic DCC systems allow you to control at least ten locos in either two- or four-digit addresses. You program the decoders individually, giving them whatever numerical address you desire (many use the numbers on the locomotives, for instance, for the sake of convenience or expedience). Additionally, standards have been established by the NMRA (National Model Railroad Association) across the board to ensure that one company's decoders will work with another company's command systems, and so on. For instance, I use the Digitrax Zephyr, an entry-level system, but I may buy decoders for my locos that happen to be made by another company.

DCC allows, as well, multiple "cabs," or individual control and throttle units that are not themselves power sources. These can be corded (tethered) or wireless. In the case of the former, "plug-in" panels are placed at various spots around the layout. In operation, disconnecting the control unit ("cab") from one panel and re-plugging it in elsewhere doesn't disconnect power from the locomotive — so you can walk around looking for another panel closer to where you might wish to be and your loco or train will continue running. The only hitch here is that you cannot change the address of either the main control unit or any external cabs or throttles "on the fly," as it were. You must let go of the train you're controlling and reselect another loco by unplugging the cab, dialing in the new address, then re-plugging in. Perhaps in the future manufacturers of these systems may find a way to allow one simply to dial in another loco's address without removing the cab's plug: the act itself of changing the address would suffice to deselect one loco and select another, etc.

DCC has allowed, in addition to all this great stuff, the introduction of on-board sound. For a premium you can now buy locomotives equipped with sound chips and a small speaker (or the owner can add these after the fact). The sounds include such things as the proper diesel rumble for whatever model the loco might happen to be (which "rumble" increases in volume and intensity as the loco accelerates and vice versa); coupling sounds; dynamic brake noises; horns and whistles (again, appropriate to the particular engine); and even wheel flange squeals . . . among other things.

This is, again, the quick and dirty low down on DCC. But as with everything digital, things can get more complex and involved. Suffice it to say, though, that DCC allows one the flexibility to "keep it simple, stupid" or grow the system as the railroad itself may grow. (Note: one may still use a DC locomotive not equipped with a decoder [address 00] but the unit will make a high-pitched whine or hum, called "singing," something that's a bit distracting, if not altogether disturbing to the ear.)

Below are snapshots of the components of my very basic Digitrax Zephyr system . . .

Zephyr's DCS 50 main unit

Digitrax UT4 tethered cab/controller

Plug-in panel (I have two on my small layout)

Panel and throttle/cab controller

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