Category: Articles

Dale Kraus

CONTROL VARIABLES (CVs): What you need to know (and don’t)

A continuing source of confusion in the DCC world is the nature and use of Control Variables. There is a dizzying array of them, 248 at last count, so it’s easy for both novice and experienced DCC users to get a bit overwhelmed. Adding to the confusion, beyond the basic CVs not all decoder manufacturers use them in the same way (!?). That’s the bad news. The good news: You can have a smoothly operating, fully functional DCC railroad with realistic motion and speed matched locomotives, and only use seven CVs!

Note: For now use 28 speed steps, not 128. Trust me…it’s easier this way.

CV 1. Engine address. This one is obvious. Simply enter the locomotive number (up to 127) in this CV and sent it to the decoder, using whatever method your system recommends… that’s it! If you wish to have a four-digit address, go to Direct Programming in your system and bring up ADR. (Some systems will say Loco Num.) Punch in the digits and enter.

CV 2. Start voltage. This sets the decoder so the loco will start creeping as soon as you crack the throttle. Enter a low value here (1 or 2) and place the loco on an operating track. Bring up the address and open the throttle. If the loco doesn’t move by speed step 3 (out of 28) go back to programming mode and increase the value by 3. Usually this will do it. Older locos with open frame motors may require a higher starting voltage. Just fiddle with the numbers until it starts slowly.

CV 3. Acceleration. This CV adds simulated mass to the loco, enabling it to start like it weighs many tons instead of a few ounces. The higher the value entered here the slower the loco will accelerate. My experience with this CV is that a value between 8 and 12 is usually sufficient. More than this and the loco will start way too slowly.

CV 4. Deceleration. Another mass simulator. This CV allows the loco to drift realistically when braking to a stop. For most model railroads, a deceleration rate of about 6 to 8 is adequate. More than this, and your crack express will sail right past the station platform. (Ask me how I know this!) A caveat here: If you are doing a lot of switching, it is best to set the deceleration to 1 or 2. Too much braking delay makes accurate spotting and uncoupling of cars difficult.

CV 5. Maximum Speed. Almost all model locomotives run much too fast. Since we’re not running slot cars, we need to dramatically lower the top speed of our locos. This can get complicated, but, as a rule of thumb, set this value initially to 30 for conventional steam and diesels and 12 for geared steam locos. After you have done this, you can tweak this value up or down for each of your locos until they all run at close to the same speed for a given throttle setting.

CV 6. Medium Speed. Set this CV to about 1¤3  the value of CV 5. This will give you a very nice acceleration curve. Tweak as necessary later.

CV 29. Configuration. This can be a confusing CV if you think about it too much. However, all you need to remember are a few simple things. First, enter a value of 6 into this CV. If the loco runs in reverse when the throttle is set to run it forward, change this value to 7. That’s all, folks! A note here: if you are running permanently coupled F or E “A” units, with or without intervening “B”’s, pick one as the “lead” and enter 6 in CV 29. The other is the “trail” and should be given a value of 7.

That’s it for now. Next time we’ll tackle the dreaded consisting conundrum.

Ken Liesse

Here’s a neat video from a 1981 documentary that I learned about from the folks down in PCR. It shows how a “prairie skyscraper” works. Pay close attention to the guy moving a box car by himself. That would make for an interesting model.

Grain Elevator Documentary

Bill Kajdzik

Bill posted some photographs from his recent trip to Stevens Pass.

Dale G. Kraus, MMR

Continuing this sporadically produced screed …

Myth #3: Track switches must be “DCC Friendly”

This is perhaps the most persistent myth in all of digital command control. Somehow, the word has gotten out that all of the all-rail-frog (i.e., Shinohara™) switches are unusable and that older insulated (dead) frog switches, such as the Atlas Custom Line™ must have their frog castings powered. Prospective DCC converts at my old shop often voiced this concern. The modeler was reluctant to take the leap into DCC for fear he would have to rip out and replace all his old turnouts. I don’t want to put too fine a point on this, but …rubbish!

This myth stems from two characteristics of the commercial all rail switch: First, since the points are connected together with all metal tie bars and a pivot bar, both point rails carry the same polarity as whichever stock rail the points are touching. This means that the frog also has that polarity. From these characteristics, a fear has developed that loco and/or car wheels will short out the command station when rolling through the points. Not possible! If both the wheelsets and the switch points are built to NMRA standards there will be a substantial air gap between the wheels and the open point rail. If you experience shorts, check the wheelset back-to-back and the point-to-stock rail clearance using your NMRA standards gauge. One of these is seriously out of tolerance.

Second, because the frog polarity changes with the point position, the open end of the frog rails must be insulated from the inner rails of the attached track. Two insulated rail joiners are the easy fix. The outer rails of the diverging tracks need not be insulated from the stock rails of the switch. Regular metal joiners may be used there. If you do not insulate the inner rails, you will have a short! It is also necessary to assure good electrical contact between the points and the stock rails. Keep the inside surface of the stock rail clean (use a Bright-Boy) and make sure the point closes completely and firmly. Be sure no bits of ballast or glue interfere.

Lastly, the Custom Line switches need not have their frogs powered unless you are using really small, two- or three-axle locos. Occasionally one of these frogs is installed one- or two-thousandths of an inch higher than the metal rails. A few strokes with a fine cut mill file laid flat on the frog will cure this.

More later.

Dale G. Kraus, MMR

Hello, everyone!

This is the first installment of a sporadic series about Digital Command Control. My intent is to be a “myth buster” and to present DCC in a non-technical, anti-geek-speak manner. Since the G-I is now in blog format, others are welcome to join in with questions, tips, and ideas for making DCC easy to understand and use. (Please note the use of “easy!”) For erudite discussions of the advanced uses of DCC, please go to the NMRA’s DCC Special Interest Group website: http://jdb.psu.edu/nmra/dccsig.html.

Onward! To kick this off, I will address two persistent myths: DCC is complicated and DCC is expensive.

Myth #1: DCC is complicated

DCC is a complex, feature-rich control system that has a vast and expanding repertoire of capabilities. Most systems come with a manual which, if read thoroughly by the newcomer, can be rather daunting. My advice: read only the “Getting Started” part of the manual and the Basic Programming section which deals with CV’s 1 through 6. Then simply replace your power pack with the DCC system. If you have two or more power packs and control your blocks with block selector toggle switches, replace one pack and throw all the selectors to the position controlled by that pack. Presto! You now have a DCC railroad. More on this in a later article.

Myth #2: DCC is expensive

For most home railroads, a basic system is all the DCC you’ll ever need. These systems have a 1.5 to 3.5-amp capacity and range from about $80 to $180. Unless you’re running large O scale locos, this will be sufficient. Basic decoders, good for N, HO and most S locos, run about $19 each. So, unless you’re running a one-horse logging show or switching pike, installing DCC can actually cost less than a comparable DC system. Remember, you have to factor in all the power packs, all the selector toggles, all the control panels, and that “spider-on-LSD” web of under-table wiring.

A Quick Tip

If you are using twin-coil solenoid switch machines, you can take the control wiring directly off of the track power buss. The track power is basically a 16-volt signal, which the solenoids will “see” as AC.

More later. Let’s hear from you all! And remember: when hooking up your system and programming your locos, read and follow the instructions! Comments are welcomed below!