4 function decoders support F0 Forward and F0 Reverse, F1 & F2. This allows for reversing headlights and two separate functions OR 4 separate functions if reversing headlights is not used.
Most Board Replacement decoders have LEDs for forward and revese lights wired to F0 Forward and F0 Reverse. They have solder pads for additional functions that must be wired by the end user.
How functions are numbered:
F0 (FWD) White
F0 (REV) Yellow
The Blue wire is Lamp Common to allow for continuous lamp operation.
F1 Green
F2 Violet
F3 Brown
F4 White with Yellow Stripe
F5 White with Green Stripe
F6 White with Blue Stripe
F7
F8
F9
and so one are each counted as one function. This is an industry wide convention.
Digitrax Decoder Series Numbers
Over the years as technology advanced, Digitrax has added more and more features to our mobile decoders. Also, over time, the industry standards and uses of come CVs have changed. Each of our decoder web pages includes a link to a specification sheet for that particular decoder so that you will know which CVs are used for which features in that particular model.
Digitrax uses series numbers to let customers know which feature set is part of a particular decoder.
The fifth character of each mobile + function decoder part number is the Digitrax series designator.  For sound + motor + function decoders we typically use the sixth character as the series designator. There are variations in a few part numbers.
Series 6
Compatible with Digitrax PX series Power Xtenders to keep locos and sound running in the presence of dirty/poor track work.*
FX3 outputs optimized for both LED and bulb applications.
FX3 outputs have configurable pulse available on all outputs.
Improved Scaleable Speed Stabilization (Back EMF).
Premium Series 6 Decoders have Scaleable Speed Stabilization (Back EMF) and usually have more than 2 available function leads.
Economy Series 6 Decoders do not have Scaleable Speed Stabilization (Back EMF) and usually have only 2 available function leads.
SoundFX Series 6 Features
Sound decoders available with either 8 bit standard or 16 bit premium sound.
Sound decoders configured for 8 Ohm speakers.
Allows for longer sound fragments.
Available in either standard 3 or premium 4 voice versions.
Auto CV configuration by sound projects
Scaleable Speed Stabilization (BEMF) optimized for sound operation.
*Some very small/specialized Series 6 decoders do not have connections for PX modules due to space constraints.
Series 5
Sound Bug Compatible: Series 5 Digitrax Decoders add the ability to add sound to regular mobile decoders with Sound Bug. Some Series 5 decoders have sockets for simply plugging in the Sound Bug while others require soldering to wire the Sound Bug to the decoder.
Series 4
First generation SoundFX decoders
Most are mobile, sound and function decoders, some are sound and function only decoders.
Series 3
Series 3 decoders with 6 functions have FX3 function outputs
Torque compensation
SuperSonic motor drive for quiet operation (silent operation)
Support for all DCC programming methods as of the time of release
Operations mode read back capability for CV read back on the main line
Scaleable Speed Stabilization (Adjustable Back EMF)
One Step decoder Factory CV reset (CV08=8 or CV8=9 to preserve loadable speed tables)
Motor Isolation Protection
Transponding
White or Golden Yellow LEDs on decoders that come equipped with LEDs
Series 3 decoders with less than 6 functions have a modified set of features
Series 3 Economy
Digitrax Decoders have all Series 3 features EXCEPT Scaleable Speed Stabilization (BEMF)
Series 2
FX function outputs
Scaleable Speed Stabilization (Back EMF)
Transponding
Series 1
Digitrax decoders have standard or configurable strobe function outputs.
