DC Loco on DCC

This section talks about running a DC locomotive on DCC powered track.  If running a DC locomotive is a must have on your list of DCC system requirements despite the risk to the the motor, then one should investigate the capabilities of the DCC system prior to purchasing.  That is what this section is all about.

Only one DC/Analog locomotive or train can run on the entire DCC powered layout.  No different than if you had one DC PowerPack/Throttle for the whole layout.

In this section are sub topics as follows:

1) WHAT DCC SYSTEM SUPPORT DC/ANALOG OPERATION
2) HISTORY BEHIND DC/ANALOG SUPPORT.
3) WHAT ARE THE RISK IN RUNNING A DC LOCOMOTIVE ON DCC POWERED TRACK?
4) HOW DOES ONE CONTROL A DC LOCOMOTIVE ON A DCC SYSTEM?
5) ARE DCC SYSTEMS REQUIRED TO SUPPORT DC LOCOMOTIVES PER NMRA DCC STANDARDS?
6) HOW DOES DCC AC WAVEFORM ALLOW A DC LOCOMOTIVE TO RUN?
7) RUNNING A DC LOCOMOTIVE WILL SLOW DOWN DCC THROTTLE TO TRAIN RESPONSIVENESS


1) WHAT DCC SYSTEM SUPPORT DC/ANALOG OPERATION

SUPPORT:
All Digitrax
All Lenz
All EasyDCC
Bachmann's EZDCC
MRC's long discontinued Command 2000 and Prodigy

NO SUPPORT:
ALL NCE
Bachmann's Dynamis.
ALL MRC systems made after Prodigy.


2) HISTORY BEHIND DC/ANALOG SUPPORT.

The history in supporting running a DC/Analog locomotive on DCC powered track was a hot topic when DCC was brand new and not fully accepted back in the early mid 90's.  DC was still KING.   The most common question people asked when introduced to DCC was:  "Can I run my  DC locomotive on it too?"  The typical concern back then was the idea the person would have to convert all the locomotive over to DCC all at once.  -AND/OR-  Switching the layout over without have a DCC locomotive ready to go would mean one could not run trains anymore until a locomotive was converted to DCC.   Installing a DCC decoder was a semi-costly task that required skills that many did not have and/or were afraid to take on and/or felt they did not have the money.  All there reasons, right or wrong, said to not take the risk and buy the DCC system at the time the purchase is to be made.   Bottom line, many DCC vendors felt they have a "ready to use option for DC" when converting the layout to DCC.  They need a temporary "bridging" solution on the way over to DCC if the DCC vendors were going to have a chance on selling the DCC system. 

Do I like the idea of supporting DC operation on DCC power track, no.  The engine's DC motor was never designed to run on AC power.   Many DCC vendors have recognized this and drop support.  But back then in the 90's one had a strong argument supporting this as a "necessary evil".  Today there is no real need to support DC/Analog as NCE and MRC have proven.  The only place it can still hold true value is in the TOY TRAIN SET market where DC will not go away.


3) WHAT ARE THE RISK IN RUNNING A DC LOCOMOTIVE ON DCC POWERED TRACK?

The short answer is potential motor damage.   No all locomotive manufacture will repair any damage to a DC locomotive that is a result of running the engine on DCC powered track.  For more infomation, go here:DC Locomotive Damage Risk


4) HOW DOES ONE CONTROL A DC LOCOMOTIVE ON A DCC SYSTEM?

Consult your DCC manual.  There is no standard method and it varies with the DCC manufacture.

Of the methods used, the most common is to use short address 0 on the throttle.  Why?  Short address zero is NOT a valid DCC address for normal DCC locomotive control at the user (Cab/Throttle) level.  However that does not mean this same address cannot be reallocated for use as the DC locomotive address at the user level.  Easy to remember and hence the popularity of its use.  To be clear, there is no DC locomotive address.  It is just a cool trick that tells the DCC system to send the throttle speed and direction commands as DC commands and not DCC commands.


5) ARE DCC SYSTEMS REQUIRED TO SUPPORT DC LOCOMOTIVES PER NMRA DCC STANDARDS?

No. The DCC standards do not require such support.  It is optional.

It has been INCORRECTLY stated that: "ALL DCC system support DC locomotive because there is a section in the DCC standards that tell you how to do it."   This sentence confuses two unrelated issues together.

1) Yes there is a section in the DCC standards that describes what has to happen at the DCC track signal level to support DC/Analog operation in such a way that does not PREVENT DCC operation.  The DCC standards must do this to preserve its primary goal of guaranteeing that all decoder will operate correctly on the track. 

2) No, there is NO section in the DCC standard that covers how the DCC system is to control the locomotive at the user level.  The DCC standard only focus on the track level and not at the Cab/Throttle system level.


6) HOW DOES DCC AC WAVEFORM ALLOW A DC LOCOMOTIVE TO RUN?

The throttle's speed and direction commands are used to  directly modifying the DCC track signal waveform timing.  This DCC waveform timing changes FOOLS the DC motor into thinking it is seeing DC.  This technique, in DCC terminology, is called "Zero Bit Stretching".  Zero Bit Stretching does NOT modify or destroy any of the DCC data in the waveform.  So none of the DCC locomotives DCC commands being sent are lost.  To learn more about Zero Bit stretching, go here: Zero Bit Stretching/ DC on DCC


7) RUNNING A DC LOCOMOTIVE WILL SLOW DOWN DCC THROTTLE TO TRAIN RESPONSIVENESS

When Zero Bit Stretching is active, it slows down the rate of DCC command can be sent to DCC locomotives.  In other words the responsiveness a given DCC locomotive has to your DCC throttle command will lag.  As you increase the speed of the DC locomotive, the slower the rate the DCC commands will be sent down the track decreasing the DCC locomotive responsiveness even further.

Many layouts have suffered operation problems when someone runs a DC locomotive on a DCC layout.  As often is the case this is a casual operating setup which means that not that many DCC locomotive might be running at the same time.   At the end of the session, sometime the DC operator forgets to stop the DC locomotive/train before layout power is turned off.   Since the next operating session may be more formal, one removes the DC engine/train while the power is off.  Upon restoring power to the layout to start a large DCC operating session, the operators suffer control responsiveness problems with the throttles.  They do not think about the DC locomotive anymore because it is not on the layout anymore.  Out of site, out of mind.  The solution fortunately is very simple, find the throttle used to control the DC locomotive and turn it speed to zero.  It just knowing what to do about it that is hard.