This is a keyboard controller, derived from the MK 88 and it has the same features as the MIDI MK 88, a midi in, midi out 4 (2 outA, 2outB) an external or internal clock complete with stop, start, continue, tempo, 2 wheels, 3 sliders, etc ...
The difference with the 88 is that the 55 has 61 notes, touch keyboard (but pleasant).
The flight case version is optional.
It has two input jack for analog two large pedals, producing a continuous control, assignable to volume, modulation, aftertouch, pitch, or other (free), plus four jack inputs, trigger type, for footswitches.
The configuration of the interface is an array: 16 values on the ordinate, abscissa 12.
two LCD screens.
The original manual in French is complete and functional.
The MIDI (midi in) is of little interest, except that extend the length of the keyboard can not merger controllers, from a Remote or a Behringer BCR 2000 by MIDI in, the MK 55, it's screen. However, the Remote can be cascaded downstream, for he, merge the signals it receives, with those it issues.
For twelve years I use it, it's never failed. Of course you have to change the lithium battery every 6 years or so. The back up is of course possible.
What is nice is that everything can be set in hexadecimal, but that it needs to be known!
Given his age, he does not know the recent changes to the MIDI standard, I do not know if the OS of this type of keyboard master may be updated (up-date): This question needs to be dug.
With the addition of a small area of control type Remote, we have the total: a 5-octave keyboard, pots and sliders, etc ... enough to control a set of important synths, collectors, hardware (20 or 30 synths, no problem).
I bought FF 3000, twelve years ago, at the bottom of the side (hey, hey) I think its current value to three hundred euros or so.