Sound-only decoder in dummy
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This may be a dumb question,...but if I have a Dummy B-Unit that has a Sound decoder, doesn't this unit have to be consisted with an A-unit in order to function ??? Otherwise how will it know to correspond the sound of it's motion with that of the A-unit pulling the train ???
Yep. You can control the locomotive directly, or as part of a consist. While part of a consist with other locomotives, then it's speed (not applicable to a sound-only decoder) and sound output (not applicable to non-sound decoders) are set matching the commands to the consist. (Note how that last statement can be applied to ALL engines & decoders.)
That's the beauty of the design of DCC - you can have 50 engines on the same piece of track but only directly control the ones you want. The dummy unit doesn't have motor control to worry about, but it will need to be consisted into a train to respond as if it is part of the train. Similary, if the B-unit was NOT in a train, you'd want it to quietly idle away on a side track, not rev up like it's in full throttle and not going anwhere.
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Also, will the Sound decoder receive commands from A-unit decoder as far as horn/bell sounds etc,...???
Function commands (like those telling the decoder to sound the horn or bell, or turn on/off headlights or other lighting features) are typically only sent to the lead unit of the consist. You have to select the B-unit or actually specify it as the nominal lead unit of the consist to play those sounds.
This is by design and logical if you think about it. On a real train, only the lead unit has the headlights turned on, and only the lead unit sounds its bell and horn. I know in your case you're just having the B-unit with sound, but in cases where you have multiple (powered, non-dummy) units all with sound in a consist, then having all of them sound their horn or bell wouldn't be right.
One trick which some people like to use is to give the B-unit the same address as the A-unit, so that any commands to the A-unit automatically go to the B-unit as well. This means you don't consist them together; whatever the A-unit does, the B-unit does. This has the effect of permanently "consisting" the two units together. Personally I don't much care for this technique as it completely ruins the flexibility of consisting any engines to run with each other as required that DCC offers, and depending on your operational style may not be realistic, as real railroads don't generally keep engines in permanent combinations (OK, early on when F-units were first introduced some American roads did get them in sets with the same number that were kept together, but that was primarily a labour issue with the transition from steam, where multiple engines meant you had to have a separate crew for each engine.) If you just have a small layout with a couple of engines, and you always run the same ones together anyway, not trying to replicate real-lift operations or procedures, then this may not be an issue for you.
