By far the most important feature of a master keyboard is the playability of the keys. If you already have some playing experience then a cheap keyboard will likely become a big issue when playing complex pieces.
The keyword here is weighted. Besides normal keys, found in every conventional keyboard or synthesizer, there are keyboards with lightly weighted or fully weighted keys. The feel of these models will be much closer to that of a real piano and your playing technique will improve from their use as well. Keyboards with hammer mechanics are modeled after real pianos and offer the closest approximation, but unfortunately they’re also the most expensive.
After the playability the number of keys is the most important aspect to consider. Fewer keys allow for a more compact design which fits on any desk but prevent a lot of octave changes with the keyboard or sequencing program. It’s been proven that if you put a piggy bank on your desk and throw ten cents in it every time you change octaves you’ll have enough for a bigger keyboard before you know it :-)
Just as important are the integrated MIDI control options, which make it possible for you to control your equipment through the master keyboard. Better equipment allows for less frequent use of the mouse or sound module. Touch dynamics are offered by all models with MIDI function but not necessarily after touch or controls of pitch bend and modulation, which can be found on any synthesizer today.
Last but not least are the included accessories. Will your master keyboard come with a sustain pedal or stand? Do you need a separate MIDI interface or just an adaptor cable? Which software is included? Can you play it out of the box or do you need to buy additional sequencing software?
If you plan to play your device on stage or in the practice room, then you should also consider features like transpose, split (different sounds for the left and right hand) and layering (multiple sounds played over one another). These functions are offered in basically every modern sequencing program but for use in a live setting it’s nice to be able to play without a computer.
Normally the MIDI Out of the keyboard is connected directly to the MIDI In on the MIDI interface and the MIDI In connectors of the individual samplers are connected to the MIDI Outs of the interface. In this case the sequencing program takes care of the data transfer to the sampler, with the advantage being that you can set the MIDI channel, octave position and dynamics directly in the program.
First, check if any MIDI data is present at the MIDI In connector of the MIDI interface (most sequencing programs provide a display, if necessary, you can also start recording and check your program's Event Editor for notes being recorded). If this isn't the problem, then check if the MIDI Out connection of the master keyboard is correctly connected to the MIDI In interface and that the MIDI interace is correctly installed.
When data enters the sequencing program it needs the ''MIDI thru function'' in your sequencer to transfer the notes in real time over the MIDI out port to the connected sampler or internally to the sound card. This function has to be on! You can usually set which MIDI channel you want to control by changing the MIDI channel of the currently selected track. Similarly, you can also transpose via the sequencing program or change the touch dynamics.
Since your master keyboard is not connected directly to the sampler(s), only via the MIDI interface, your sequencing program is needed to transfer the played notes to it or the built-in sound card. Many MIDI interfaces offer the option of transferring the MIDI data directly when the computer is off if they don't do it automatically. For others though you will need a MIDI adapter (just connect MIDI In to the interface via the adapter with MIDI Out).
If you would like to use the built in sounds of your synthesizer or the sounds from another sampling device (sound modules, sound card etc.) in your sequencing program, then you need to set your synthesizer to ''Local: Off'' mode. This function is normally located in the basic settings of your synthesizer (where you can also find the MIDI send channel). In ''Local: Off'' mode, pressing the keys produces no sound but the notes will be transfered like normal over the MIDI Out port to the connected equipment (probably the interface in this case). However, signals at the MIDI input of the synthesizer are played back normally.
If the function ''MIDI Thru'' is active in your sequencing program then the data will be automatically trasnferred to the connected sampler, sound card or to your synthesizer. You can then set the output or the MIDI channel and choose which notes are transferred to your sequencing program, and your synthesizer will only play them once the data is sent back to it.