For the purpose of discussion throughout this site, a loco that does not have a DCC decoder installed is called an analog loco, and a loco that has a DCC decoder installed is called a DCC loco. A layout controlled with a traditional DC power pack is called an analog layout, and a layout controlled by a DCC systems is called a DCC layout.

1) Controlling a DC analog loco on a DCC layout
2) Controlling a DCC loco on an analog layout.

This page dwells on controlling a DCC loco on an analog layout. Refer to Analog Control for information about controlling an analog loco on a DCC layout.

Most decoders have "Automatic Analog Conversion". This mean that the DCC loco with this feature can run on either a DCC layout or analog layout without having to do anything more than taking the loco from one layout to the other. The decoder accomplishes this with a simple algorithm that can distinguish whether or not the power it's getting includes DCC signals.

DCC control packets have a certain progression. They first have a stream of "preamble" bits followed by the address, instruction, and error detection bytes - then back to the preamble for the next packet. Decoders that have the analog conversion feature look for this when powered up. If this rhythm of data cannot be identified, the decoder assumes that it is sitting on an analog layout. When an analog conversion decoder is powered up on an analog layout, it simply passes track power straight though to the motor.

A certain amount of voltage needs to be on the track for an analog loco to start. This amount varies depending on the loco and its motor. However, a DCC loco needs to have slightly more voltage on the track for it to start. This is because there is a voltage drop when the power goes through various electronic components in the decoder. The bridge rectifier alone will drop the voltage by 1.4 volts. So if a loco normally starts at 1.6 volts, for example, it will require at least 3 volts for that same loco to start with an analog conversion decoder installed.

When running a DCC loco on an analog layout, it should be done with an analog throttle that does NOT have pulse power. The pulse that the throttle puts out could confuse the decoder into thinking it may be DCC, only to find out that it isn't. Then again thinking it is, then isn't. The result could be a loco that will spit and sputter and not run. Also, if a loco doesn't run smoothly even when using a non-pulsing throttle, the momentum programmed into the decoder could be messing with it's mind. If you encounter this, simply program momentum out of the loco before taking it to the analog layout.

While you won't have Function Control to turn lights on and off when running on an Analog layout, some decoders allow you to program the decoder to have the lights either on or off when running there. And if the decoder allows for special FX Lighting effects, any lighting effect programmed into that function will also work on the analog layout. This means that if you have a loco with a Mars light programmed in, as soon as the decoder gets enough voltage to work, the Mars light will start to work. Refer to DC Function Control for more information.

Automatic Analog Conversion can be turned on or off. If turned on, it will convert from DCC to analog and back automatically. If turned off, the loco will not run on an analog layout - it will only run on a DCC layout.

There are several reasons why one might not want DC analog conversion to work - - such as eliminating the little jump some locos make when some systems are turned on in some configurations. This little jump is not prevalent. Since most people have not experienced this little jump, most people don't even know about it and this fix.

If a Digitrax decoder (one with Analog Conversion turned off) is running on DCC, then suddenly is switched to DC power, the decoder will use the deceleration momentum programmed into it to bring the loco to an orderly stop. Then if DCC power is turned back on, the loco will start up according to the acceleration momentum and resume speed. With this feature, you can easily rig stopping tracks for red signals.

By feeding power to that section of track through a relay that is controlled by the same wire that controls the red signal, you can feed that stretch of track with DC voltage or DCC power, according to the signal. When the signal is green, the relay allows DCC power through to the track. When the signal turns red, it will trigger the relay to switch track power for that section from DCC to DC. When a loco with Analog Conversion turned off gets onto that section of track, the loco will slow to a stop. When the signal turns green and restores DCC power to that section of track, the loco will start up again - all automatically.

Doing this requires you to feed DC power to the relay through a pair of 1157 automotive bulbs. With these bulbs, the two power sources won't fight each other when the loco crosses the gaps between DCC and DC.

Note that to make this work the decoder MUST have the Automatic Analog Conversion feature, and have it turned off. If it doesn't have the feature, the loco will stop instantly when it hits the DC voltage, and start instantly when DCC power is restored.

Atlas is now shipping locos with decoders that are called Dual Mode. This means that they will operate on a DCC layout, and will also operate on a DC analog layout without the usual drop in voltage like decoders with "Automatic Analog Conversion" have. There are advantages and disadvantages.

The major advantage is that when running on a DC analog layout, there is no voltage drop. The loco will operate exactly as it would without a decoder. I assume they opted for this because they are now (at the time of this writing) designing new HO scale locos to have decoders already installed. And since you can't buy one of these locos without a decoder, the decoder must allow for normal DC analog operation. Another advantage is that you don't have to install a decoder when you get the loco.

The disadvantage of this method is that it is not automatic. As such, it will not operate properly on a stopping track as described above. It also does not have the capability to allow you to have functions on or off when operating on an analog layout - all functions are on and that's it. Lastly, if you want to introduce other lighting and features that the decoder does not come with, you have to remove that decoder and replace it with one that has the features you want.

If you would like to have the advantage of running on an analog layout without the voltage drop and still have all the advantages that Automatic Analog Conversion provides, you can wire your loco's motor to by-pass the decoder when operating on an analog layout.

Do this with a 2x6 socket (or plug) and 2x4 plug (or socket). Mount the 2x6 connector in a place that's easy to get to. Wire it with the motor leads going to the middle connections, track power to the two pins on one end and decoder motor power (orange and gray wires) to the two pins on the other end. Jump two pins of the 2x4 together, and the other two pins together. This makes a parallel jumper. When plugged onto the 2x6 connector spanning the decoder/motor connections, it will operate on a DCC layout as normal - stopping track and all. When spanning the track/motor connections, the loco motor will operate on an analog layout without any voltage drop, but the decoder functions will still work - Mars light and all.

When you take the loco from a DCC layout to a DC layout, or vice versa, you have to change the jumper. So you should put the jumper in a place that is easy to access.

This is strictly a decoder feature and does not require anything in the Command Station for this feature to work. If the decoder has this feature, it will work on all DCC and DC analog layouts.

Throttle Up! SoundTraxx decoders do not have this feature - obviously, because you wouldn't have Function Control with DC Analog to control the sounds or crane.

All other locomotive decoders that Loy's Toys carries have this feature. Automatic Analog Conversion is enabled as it comes from the factory - but is not intrusive in most cases. You do not need to bother with it unless you have a specific need to turn it off, such as Analog Braking.

Automatic Analog Conversion is not intrusive. You don't have to know or think about it or learn about it unless you want to use it in some way.

Bit 2 of CV29 is used to turn this feature on and off. Therefore, this requires a DCC system or other method to program CV29. Refer to CV Bit Manipulation for more information.

All of Digitrax's current systems can program any legitimate value into this CV.