ESU
really has four product lines for Sound Decoders; Select, v4.0, XL and M4.
The physical decoders come in three basic sizes; Micro, Standard, and XL.
The same hardware is used for the Select and the v4.0, both Micro and Standard. The only difference is firmware. Both models are fully compatible with the DCC protocol.
The Select Direct uses a Micro decoder mounted to a special adapter board. The adapter board is made to be a drop-in replacement for the original circuit boards in Athearn, Atlas and other models.
The M4 uses the same circuit board as the v4.0 but there are differences in components and this model incorporates the Marklin Motorola M4 protocol in addition to DCC.
The XL is physically larger to accommodate the larger components required for the higher power rating.
The basic difference between the physical decoders is the physical size and the motor rating.
Physical size...
Micro - 1.00x0.42x0.15 Inch (25x10.6x3.8 mm)
Standard - 1.24x0.62x0.26 Inch (31x15.5x6.5 mm)
XL - 2.04x1.60x0.56 Inch (51x40x14 mm)
Motor rating (continuous)...
Micro - 0.75A
Standard - 1.1A
XL - 4.0A
Function outputs...
Micro - (4) at 250mA each plus (2) logic
Standard - (4) at 250mA each plus (2) logic
Select Direct - (6) at 250mA
XL - (12) at 500mA each plus (4) 5.0V servo
There are some variations with regard to protocol also but all models are DCC compatible.
For those who don't know, DCC is a protocol. So is Motorola, M4, Selectrix, MTH DCS, and Lionel TMCC.
A protocol is basically just a language like English, Spanish, German and Italian. The are all saying the same thing, speed up, slow down, they just say it differently. If the decoder doesn't speak the same language as the command station, it won't know what to do.