The Purpose of Function Mapping

There is an NMRA standard for mapping decoder functions to buttons in DCC, and these are supported by all CT Revision 4 decoders too. However, the NMRA standard only defines how the first few functions can be mapped, as it was developed before any decoders appeared on the market with many more functions. The SL51-4, for example, provides 8 physical function outputs which can be used for things like lighting, smoke units, couplers/decouplers and just about any other electrical device you can dream up which could fit in the train.

On top of those physical function outputs, you also have the callable sound effects which you may want to control with the press of a button. There are up to 16 such sound effects available for this purpose, plus you need a button to switch the main running sounds on and off.

Just to top it off, Function Keys can also be assigned for other specialist purposes, such as the hidden Low-Beam feature, which causes temporary dimming of lighting outputs for a timed period when an FKey is pressed! There are one of two other hidden features akin to this too, but we won't go into detail here!

So, in total if you need a lot of function keys! This means CT have had to provide some non-standard CVs to allow for a wider mapping range.

To help you calculate the correct values for the desired function mapping, you can use the table below. Each Function Key has 2 CVs associated to it, and by setting the corresponding bits in each of those 2 CVs you can define which functions/sounds that key controls. A single function key could control multiple actions if you wanted it too, such as synchronising a firebox light with the sound of coal shovelling.

For decoders that have less function outputs (SL76 has 4 for example), thesecan be mapped in exactly the same way, although obviously there is no point in setting any functions to activate the non-existent outputs!

16 Effects, or 8 Effects?

CT have implemented the 16 sound effects in a rather interesting manner, which is good for controllers that only have 12 function buttons. Basically there is no way to use anything beyond F12 with the CT decoders which seems rather limiting until you discover the 'ALT' function. ALT is a special function that can be assigned to the function key of your choice, and allows you to switch between 2 banks of 8 sounds.

So, as an example let's say you assign ALT to the F4 key... you could then map sounds 1-8 to keys F5-F12, giving access to the first 8 sound effects, but with F4 switched on, means that F5-F12 access sounds 9-16 instead! Brilliant!

Downloadable PDF and Word DOC formats of this table, with an example mapping can be obtained at the bottom of this page.

HINT: For document links below, either open them directly in your web browser by clicking the links, or right-click and save target instead...

Download PDF of SL51-4 Extended Function Mapping