Sometimes, it is advantageous to install dual decoders in a loco, usually one for the motor and one for sound. Dealing with dual decoders CAN be a hassle, but there are typically several ways to deal with it.
The problem is that when two decoders are both wired to the track, programming the individual decoders can be an issue. If both decoders happen to respond to the same CV, then trying to read the contents of that CV can be ambiguous because it is hard to tell which one, or both, is doing the talking. Sometimes it is necessary to program different values, such as addresses, into one decoder and not the other. This can be tricky.
There are several ways to deal with this issue.
Switches. On large scale locos, there is often room to put two SPST switches in series of the track leads of each decoder. That way, one can easily be taken off line. This doesn't work so well on small scale locos.
Physical Separation. On steamers, it is sometimes possible to mount the motor decoder in the loco and a sound decoder in the tender. If BOTH units pick up power from both rails, this is an easy solution. Simply disconnect the units and place the one that you want to program on the programming track. This also works with a powered A unit and a dummy, sound equipped, B unit.
Decoder Locking. SOME decoders support a feature called Decoder Locking. This feature is controlled by the values in CV15 and CV16. BEFORE the decoders are installed, or when they are individually accessible, write a value into CV16. This is the "lock." The value of CV16 is usually between 0 and 7. CV15 is the "key." When the value stored in CV15 matches the value stored in CV16, the decoder is "unlocked" and programs normally. When CV15 and CV16 are NOT the same value, the decoder will not allow programming and will not respond on a programming track.
Normally, both values are zero and the decoder is unlocked. If the decoder supports locking, then to use it, program the lock first into CV16 and then program the key into CV15. JMRI supports this feature for newer Digitrax decoders in the "Digitrax" tab. Generally, the type of decoder is defined in CV16. A motor decoder is usually assigned a value of 1, a sound decoder a value of 2. The ONLY CV that will work when the decoder is locked is CV15 so that when you define a lock in CV16, you are still able to change the key in CV15 and neither will respond to any programming except another change to CV15. To lock BOTH decoders once they have had a lock set, program 0 into CV15. If both decoders are on the programming track at that time, both will lock. To program one of the decoders, write the "key" for that decoder into CV15. The decoder where the key doesn't match the lock will remain silent.
Consists. An easy way to deal with the problem is to simply consist a motor and sound decoder provided that they have been assigned different addresses. I usually make the sound decoder the "top" address so that the function buttons will drive the sounds without having to actually dial up and select the consisted address. This method works quite well and when it is time to do some programming, do it in OPS mode on the main which is directed to a particular decoder by it's address. This method is the most practical one where one of the decoders does not properly support 4 digit addressing. The Phoenix P5 "DCC" sound system in particular doesn't like 4 digit addresses so I use 2 digit addressing with the motor and sound decoder consisted in this case.
Advanced consisting is another way to program a motor and sound decoder to respond to the same address without having to set up a consist when the set is moved to another layout. However, advanced consisting is a pain AND the consist address MUST be in the range of 0 to 127. This is discouraged on the LAMRS layout due to the potential for address conflicts.
Address Tricks. Decoder locking is not supported by many decoders and BOTH must have it for decoder locking to work. This has been the case EVERY time I have installed dual decoders, one or both do not support decoder locking. So in some cases I use a consisted sound and motor decoder and other times I use an addressing trick which I think actually works better and is less confusing. If I want the combination to eventually have an address in the 2 digit range (1-127), I usually use a consisted address. If I want to use a 4 digit address for both, then I use this address trick.
If you normally use 4 digit addresses then you want to assign the motor and sound decoders DIFFERENT 2 digit addresses and the SAME 4 digit address. Set up CV29 to use 4 digit addresses (add 32 to the base value). The two digit addresses in the decoders will be ignored and the pair will use "basic" consisting (same address) on the 4 digit address so that the pair are easy to use and transport to other layouts without hassle. When you want to program the decoders, use OPS mode programming or service mode programming to set CV29 for two digit addressing (subtract 32). This then causes the motor and sound decoder to "split" apart and pick up their different 2 digit addresses. Then consist the two digit addresses so that the sound and motor work together again. Then use OPS mode on the main to make whatever changes that you want to either decoder by programming to that decoder's 2 digit address. When you want to rejoin the decoders to their common 4 digit address, either write the original value of CV29 back into both decoders individually in OPS mode or together on the programming track in Service mode. Using this address trick allows the decoders to normally operate together like a consist AND to retain that "consisted" operation when the loco is moved to another layout and to retain the ability to use a 4 digit address.
Note that this trick can also be used with a primary 2 digit address by just reversing the process. The 2 digit address becomes the "joined" address and separate 4 digit addresses are used for programming.
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© 2010 George Schreyer
Created 28 Feb 10
Last Updated December 2, 2010