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Roco sound fitted German Railbus 798/998


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My friend Peter Wurcbacher and I are building an HO German layout, Eideltor, for the exhibition circuit, all DCC sound. As part of the stock we have acquired 2 Roco 798/998 Railcar and trailer sets, sound fitted. The other evening I decided it was time that address 3 was consigned to history, and the two railcars addresses amended to 798 and 799 respectively. With power car and trailer together on the programming track I could change the power cars to the appropriate address, but not the trailers. Eventually, on a whim, I decided to try a two digit address. 98 worked for the first set, including the trailer and 99 for the second set, also including trailer. Have Roco used a cheap chip that only does 2 digit addressing? Why? Has anyone else any experience of this? Trevor

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The sound decoder is most likely a ZIMO (reading CV8 = 145 will confirm), and therefore will accept 4 digit addresses. Is there a second decoder in the trailer, or does the unit have Roco's power transmitting coupling system and only one decoder per unit?


What is the value in CV29?


What DCC controller are you using?


Kind regards,



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Hi Paul, thanks for your input. 

The model is a powered railbus with a non powered driving trailer. Both are fitted with lights. The coupling is plastic and no electrical connection is made. Each vehicle is fitted with a decoder, Zimo in the power car, no idea about the trailer. I am using a Gaugemaster Prodigy Advance controller.


With both vehicles together on the Program Track I changed the address to 798.  The address of the power car changed and I was able to control it in the normal way. The appropriate change was made automatically to CV29 for 4 digit addressing. The address of the trailer car did not change.  I know this because the lights didn't work on address 798 but continued to change on address 3. If I remove the power car from the track I am unable to read anything on the trailer.


Again with both vehicles on the program track I changed the address to 98. This time both vehicles accepted the address which leads me to the conclusion that the decoder in the trailer will not accept an extended address like some of the early Hornby and Bachmann ones, as suggested by Corax67.


As for Legomanbiffo's answer, for which thanks, my Lenz 100 seems to program addresses 100 to 127 differently to NMRA standard. As Lenz was supposedly used as the standard I find this puzzling. 


We can manage perfectly well with a two digit address in this instance. I am just puzzled as to why Roco should skimp on a decoder in such an expensive model. Thank you all once again, Trevor

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It may not necessarily be a case of skimping - as it is in an unpowered vehicle then Roco have installed a "function only" decoder and these are generally lower spec than their motor controlling counterparts.


The reason for fitting a "function only" decoder is that without a load (motor) then a motor controlling decoder will not programme at all on a track thus removing the opportunity to install a pair of matched decoders for the manufacturer.

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Reading the thread,  I think the likely reasons for the observed behaviour are:

A combination of trailer car lacking a sufficient load (typically a motor) and the Gaugemaster/MRC system trying to be "helpful" and failing at that attempt.


If a decoder hasn't got a fairly high load on one of its outputs, it cannot send an "ack" pulse back when programming.  This pulse is necessary for reading a decoder (hence unable to read it, and systems use the pulse to confirm that a write has worked on the programming track.   And then some systems will abort sequential CV writing sequences if the "ack" isn't received.   


When setting a 4 digit address, there are three CV's to set; CV17, 18 and 29.  These are handled by most systems for you, avoiding complicated calculations for the user.  BUT, if the system is waiting for the "ack" after sending the CV change for CV17, it will never send CV18 or CV29, and thus the programming is incomplete.  



a -  program on the main line,  but setting addresses on the mainline is difficult (or even impossible) in some European chips. 

b -  program each CV one at a time, but this makes four digit addresses awkward because you have to calculate the appropriate values for CV17 and CV18 (there are numerous online calculators to do this, including one I wrote on the 2mm website).   Whilst the system may report "decoder not found", the chances are that the decoder will have accepted the change, just not sent an "ack" pulse.

c -  temporarily add a motor load to the decoder.



- Nigel

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