Backwards compatibility of DCC systems

[idlemarvel]

Having just been through an exercise with a fellow forum member to transfer a loco from one DCC layout to another, I wanted to relay the few pitfalls we encountered.

The “source” layout uses Lenz kit with full range of speed steps, function keys serial and parallel, running at 19v.

The “target” layout uses first generation LGB MZS/MTS kit running at 24v.

The loco in question was a standard ready-to-run LGB loco with Massoth XL decoder and SUSI sound module.

Problem one was anticipated, changing the number of speed steps from 28 to 14 as that is all the early MTS can handle. If you don’t do this you get many strange effects including lights coming on and off as you increase speed. Easily fixed by turning off bit 1 of CV 29 (NMRA configuration CV)

Problem two should have been anticipated but wasn’t. Target could not get sounds to play (function keys 1-8). Source was using parallel commands, target can only issue serial commands. For example, pressing F8 just sent F1 command 8 times. Again, easily fixed by turning on bit 0 of CV 49 (Massoth configuration CV)

Problem three was not anticipated. The light bulbs kept blowing when running the loco on the target layout. On the source layout the lighting voltage (CV 50) was set to 8 meaning 8/32 of the track voltage of 19v or about 5v. On the target 8/32 of 24v comes to about 6v which was just enough to blow the bulbs. CV 50 was changed to 3 and all is well.

I think all of this is well known but easy to forget.

 

[Zerogee]

By "first gen" MTS you mean the Lenz-built (I think?) kit with the "loco mouse" controller, before Massoth got involved with MTSII?
The bit I'm really surprised about is the comment about the bulbs blowing..... I really wouldn't have thought that the extra volt or so would have been enough to do that. Very bright and with possibly shortened overall life, maybe, but not blowing out frequently.....

Having said that, I did manage to blow all the bulbs in my DR black Ts3 despite having carefully ensured that I dropped the lighting voltage of the LS decoder I'd installed down to about 5-6v.... not realising that the bl**dy bulbs were 3v..... and to make things worse they are not easily replaceable like LGB bulbs, I can't even figure out how to get the lamps open to get at them.
When I later put an LS in the green Landerbahn version I did what I should have done with the black one, and simply left the factory voltage stabiliser in circuit rather than removing it. Hindsight is a wonderful thing.

Jon.

 

[Neil Robinson]

A useful reminder, thanks.

I'd like to add a couple of other comments.

Make sure that the decoder is what you think it is before making changes. it's not unknown for different decoders even from the same manufacturer to use different CVs for the same function.
Also, and probably most important of all, ensure all changes are reset to their original values before the loco returns to it's home layout. It's not that long ago that I managed to sort a loco that the owner was on the verge of selling. Despite liking the model a lot it would no longer work on his layout after some well meaning individual had altered some CVs to allow it to run on a club layout.

 

[Greg Elmassian]

The lights blowing is one reason I never use PWM to dim lights, only resistors or current limiting devices like a CL2... DCC system voltages can vary quite a bit, many people run one type of loco that does not require the full 24v on the rails.

The other reason I don't like PWM is because it is very seldom "linear" in terms of the settings, often reading the decoder specs carefully, there is no guarantee of voltage, but just of the PWM percentage.

LEDs and incandescent bulbs are nonlinear in their response to voltage, so you can run into issues.

Furthermore, often people do a reset, wipe out the pwm setting, give full voltage to the bulbs/LED and they blow out.

So, I often take what people think is an overly conservative or too safe way to do things, but rarely burn things up.

So another thing to consider when doing installations, of course I understand these were existing installations.

Greg

 

[stockers]

I tend to use 5 as a value for CV50 (Massoth, other makes may use this CV very differently). It keeps the voltage down whatever the input and gives a slightly yellow glow which i think is more realistic.

 

[Dan]

NOTE MTS II was initially Serial only!!!! It must have a P sticker to do parallel!!!

Also, hit F and 9 to switch between serial and parallel on all MTS systems. Why is this needed?? Older 55020 LENZ decoders were serial only.