NMRA Conformance

The following is a discussion about NMRA DCC conformance (The issuing of a conformance warrant to a given decoder) and the many issue that it faces today.

HISTORY: The NMRA put together in the 80's a set of DCC standards (S) and recommended practices (RP's) regarding the operation and programming of decoders. One of the key goals of the DCC standards is the concept of guaranteeing the mobile decoders installed in a given engine will work on any DCC system at the track level. Given the majority of the investment in DCC will be the decoders, the standards directly addresses the need for preservation of this investment. The enforcement mechanism is having a manufacture get a "DCC Conformance Warrant" to claim compatibility with DCC as blessed by the NMRA. The Conformance process involved the DCC product be tested against the NMRA against the standards, RP's that ware implemented (CV level testing). It further tested the electrical performance limits regarding real world poor DCC signal reception due to noise and voltage spikes. The presence or lack of a conformance warrant would allow a purchaser to make an informed decision regarding a given decoder purchase and the risk associated regarding compatibility. These standards succeeded and allowed DCC to take off.


1) Some people think if a decoder is missing some CV's, it is not a DCC conforming decoder. Sorry. As it turns out, most CV's are optional under the standards. Hence the decoder is conforming. The reason behind this optional CVs at the time the standards were written was to keep cost down by not forcing the DCC manufacture put in more design effort and hence more cost into the decoder.

OBSERVATION: What is true today is this is no longer a valid argument to design a decoder that just meets the minimum DCC standards. Why? Decoder design has matured well beyond the point of basic DCC compatibility. If a DCC manufacture does not introduce competitive features, you do not have a real product and can be ridiculed in the market place. Example: Early MRC decoders suffered this problem. Compatible, yes. Feature competitive...no.

2) Some people simply think that since the decoder does not work on their layout with their setup, it incompatible with DCC. The rationality is that since other brands decoder work, it the problematic decoder fault due to conformance issues. Sorry. DCC conformance do NOT guarantee every decoder will work on every layout built, it only guarantees it will work with every DCC system as defined by the NMRA testing setup and procedures when used on the most common size layout at best. The problem is often traced to something electrically related to the layout but remains a mystery unless one knows what to look for and where. The electrical solution is often a very simple to fix.

OBSERVATION: What is the common size DCC layout? A 4x8 layout is used in the conformance testing of DCC systems and as it turns out in a report by Model Railroader magazine or equivalent report I read, it is the most common size layout. So by far most people do not need to care much about conformance issues at the electrical level. The wiring is so short, it almost certain it will work.

Great. But what about large layouts? It was stated: "The DCC conformance test only cover slight levels of DCC signal distortion at the track level." Unfortunately when the layout gets large, there are many background variables that start to make their presence known in the operation of the layout. Issue such as the quality of the wiring (Wire gauge or or poor wiring), electrical noise present (lot of engines running at the same time) and the DCC system being used (characteristics of the original DCC signal from the booster) on the given layout. The unique combination can potentially be problematic such that it causes an operational failure while using a conforming decoder. I have personally seen a full conformance decoder (Lenz) have an operation problem on a large layout first hand. An investigation with an oscilloscope shows a lot of DCC signal distortion. The solution involved changing the layout wiring to clean up the DCC signal. One could suggest that the conformance testing need to change to reflect the needs of large layouts. However, this will increase the cost of the decoder for small set of special need layouts. That is not vary fair to the 4x8 layout guy who will have to help pay for this and in effect subsidize the needs of a few. Besides DCC is often sold as "easy to install" and any special wiring requirement would defeat that message when in fact the majority of the layout are small. The greater good wins. That said, what do large layout do? There is no wiring standard or guideline for designing a large layout wiring system so it will work as reliably as a small layout will. Like the foundation of a house, a unreliable foundation means the house can fall down. In DCC terms, one can potentially get unreliable decoder operation.

To learn more about how to do better large layout wiring, see: Large Layout Wiring


SHORT ANSWER: YES. BUT it requirement has been reduced greatly. The conformance warrant does not mean as much as it use to for a lot of reasons. Examples:

1) Very few TRUE NMRA compatibility/Conformance issues today.

Decoder conformance discussions fall into two categories. Brand New Design and a Mature design.

1a) Brand New Design.

Here is clearly where the TRUE value of conformance testing is found . Brand new decoders that has never been tested before. Didrik Voss, NMRA conformance chair, said this on 6/10/13:

"As you can see, it is very easy to get a conformance Warrant based on CVs if the manufacturer follows that Standard as it is currently written. The key problem is to ensure that the decoder can read the DCC signal that is slightly distorted on the track (motor noise, poorly generated DCC signal from a Command Station, Railcom, Transponding). This series of tests take 6 days (and nights) per decoder to run. We are currently running two sound decoders through this series of tests and they are having problems."

OBSERVATION: Given the huge effort the DCC manufacture must put into the software to make is pass the test, the DCC manufacture will avoid touching this portion of the software that deals with compatibility. If it ain't broke, DO NOT FIX IT! If you touch, you must re-certify.

1b) Mature Design: Post NMRA cerfitication, the decoder software regarding effects NMRA compatibility does not change in the decoder as new version of the same product line are introduced. The portion of the Decoder software that does change are all about new features and functions that have nothing to do with NMRA conformance. In theory the decoder is just as compatible today as it was back when it was certified. Didrik Voss, NMRA conformance chair, said this on 6/10/13:

"The manufacturers do not see the value in getting an NMRA Warrant, which tells the customers that the decoder adheres to all NMRA DCC standards because you, the customer, do not insist they get a Warrant before you purchase the decoder. As soon as the manufacturer sees that sales will increase because of the Warrant, they will get a Warrant."

OBSERVATION: Lenz has been in the US market for a longest time and was one of the few DCC vendors that constantly got conformance warrants for its decoders. They pushed this hard in their advertising. None the less, they have continued to lose US market share over the past decade while other DCC vendors have entered in the US market during this time and have grown. Granted there are other factors involved but clearly having the NMRA conformance does NOT appear to not be such a compelling reason to buy Lenz and avoid other manufactures.

2) The DCC standards do not keep up with the progress of DCC decoders.

The following issue SHOULD BE A CONFORMANCE issue because it clearly effect operation at the track level which is what the DCC standards are all about. The lack of tackling these obviously daily problematic issues head on undermines the importance of having the DCC standards and RP's.

2a) Incompatible "Sound" and "KA/StayAlive" Decoders on the Programming track.

Didrik Voss, NMRA conformance chair, said this on 6/10/13 summarizes the whole issue:

"NMRA standards establish the maximum amount of energy on the programming track to 250mA for no more the 100mS. The reason for limiting this energy is to ensure the decoder does not burn out if it is miss-wired to the locomotive. However, this amount of energy is insufficient to charge the capacitors typically used with sound decoders and now with the KA/Stay-Alive decoders. If the decoder is programmed on the Main where the power is always on, the capacitors are always charged and they do not interfere with the DCC commands.

The Soundtraxx booster PTB100 overcame this problem on the programming track by extending the time power was provided to the programming track. This works if the capacitors are instantly charged. However, for the KA/Stay-Alive decoders where charging may take several seconds and the PTB100 does not take enough time to charge the capacitors before the DCC command is sent to the decoder. Slowing down the charging of the capacitors is a good thing on the main. If anyone has QSI sound decoders, you probably know not to put too many of them on the layout. If there ever is a short and the command station momentarily shuts down then turn back on, the charging of the QSI capacitors will look like a short (in-rush current) and the command station will continue to shut down over and over again. Slowing down the charging of the Keep-Alive capacitors solves this problem - but it now may be a problem on the programming track as explained above.

I plan to look into this problem in more depth over the weekend. Possibly NMRA will need to change the standards to overcome this problem."

Observation: The problem has been around for a long time starting with the very first SoundTraxx sound decoders sold in 1998. They would not work on the programming track very well because of the high inrush current issue with capacitors. To address the issue, SoundTraxx and other vendors introduced a Programming Track Booster. True this is a solution but ALL at the customers expense. Instead of taking charge and updated the programming track power specification head on, the DCC committee never address the issue. Strike one.

The high current inrush problem expanded greatly when Broadway Limited Imports (BLI) introduced the first HO engines with factory built in sound decoders from QSI in the first half of 2000. The problem went from the programming track to short circuit recovery problems on the main line in a very little time. Just like the programming track booster, DCC manufactures where forced to address the issue resulting in again more cost to the customer. Yet another opportunity to take charge of the situation was again lost by the DCC committee. One begins the question why have a DCC committee that does not do anything especially in matters effecting every day basic operation of the DCC layout. Strike two.

in 2013, we have taken the capacitor issue to a new level with the use of Super Capacitors in decoders with marketing names of "Keep Alive", "No Halt Insurance" or "Power Modules". DCC decoder manufacture knew early on that these products were going to have a big high inrush current problem and have made efforts to address the issue. But again without a standard to guide the DCC manufacture, they are left to come up with there OWN solution which does not guarantee it will work with every DCC system. Strike three.

2b) Lack of standardization of the most common and important sound decoder functions. Bell, Horn and Sound Mute. When sound decoders first appeared, the opportunity to define some standard for these most common functions slipped by. I suspect there was the false fear that the NMRA would be interfering with the progress of technology. What the NMRA did not do, was latter addressed externally by an agreement between DCC sound decoder manufactures WITHOUT the DCC committee being involved. What does that say?

3) The US effort to move forward with the DCC standards has collapsed.

Some of the reason are political and some are practical. Ask a given US manufacture why, and they will give you reasons. I know from my NMRA member participation on the DCC standards that things happended that should never have happened the way it did undermined the process. This lead to politics entering into the scene and consequently went a direction that no one liked. I lost all interest after that and likewise many of the DCC manufacture no longer wanted to participate in the open working group. There was some effort to start a different group that was only open to DCC manufactures. Didrik Voss, NMRA conformance chair, said this on 6/10/13:

"The DCC working group is functional – in Germany. Since the US manufacturers elected on discontinue their participation in the WG, the European manufacturers elected to hold the meetings in Germany. We continue to take recommendations from this group in changing and upgrading our standards."

Note: All quotes from NMRA conformance chairperson Didrik Voss are from a series of emails posted on the TCS Yahoo Group June 7th to the 9th 2013.