Well, I'm no expert on the topic, but let's see if we can just put down some basics and see how one would start off, even if it is from scratch...
MIDI is fairly basic. We have a maximum of 16 channels IN or OUT that we can use. We assign a channel to a function, so that function can trigger a sound on whatever the destination device is (arranger, module, etc...). Your first job is to learn what channel number is used in what area.
The first link in the chain is likely going to be the item that sends signals to an arranger. That can be a MIDI keyboard, MIDI sax, guitar of any of a dozen other possibilities. For our use I'll make references to my accordion, but each person may need to look at and learn about their setup. Basically you will want any device to send signals for the left (2 channels, one for the bass, one for the chords) and right hands (1 channel), as a minimum. The MIDI signals originate here and move OUT to the next step in the chain, so we are going to want to document what those MIDI OUT channels are.
As an example, the FR-8x uses these MIDI OUT channels for the following functions:
MIDI OUT channels
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Accordion section Ch. 1
Bass/Free Bass section Ch. 2
Chord section Ch. 3
Orchestra1/Organ section Ch. 4
Orchestra Bass section Ch. 5
Orchestra Chord section Ch. 6
Orchestra Free Bass section Ch. 7
Drum section Ch. 10
Orchestra2 section Ch. 11
Basic Channel (For selection Sets, User Program) Ch. 13
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As a bare minimum, you will want to find out what channel is used for the right hand (in my case CH1), left hand bass ((in my case CH2) and left hand chords (in my case CH3). With this info we can now connect a MIDI cable from the MIDI OUT connector of our accordion to the MIDI IN connector of our arranger and then configure the arranger to listen on those specific channels. For example, if all we wanted to use was an external sound module, we set it to "listen" to CH1 so that anything I play on the right hand is sent to the external module and comes out in the sound/instrument we have configured on the controls of that external module.
If you are using something like a Ketron arranger that is a rhythm and accompaniment unit, you will want it to listen to CH3 (or whatever channel your setup uses for the CHORDS) and then as you change chords, the chords used for the accompaniment of the arranger follows along. You can also set it up to listen to CH1 and choose from a selection of instruments that would be played as long as it is configured to listen to CH1, of course.
If you have something more complex like a BK-7m, you have more choices and channels to map to specific arranger functionality and a BK is very configurable. For example, there is a wizard that can completely configure the BK-7m to match the complexities and extra channels of a Roland V-accordion in a few seconds, and then it's configuration matches up with the following functionalities of the V-accordion (mentioned above), thusly:
MIDI IN channels
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UP1 Ch. 1 Treble
UP2 Ch. 4 Orchestra
LWR + NTA (Note-to-Arranger) Ch. 3 Chords
MBS + NTA (Note-to-Arranger) Ch. 2 Bass/Free Bass
NTA (Note-to-Arranger) Ch. 5 Orchestral Bass
NTA (Note-to-Arranger) Ch. 6 Orchestral Chord
NTA (Note-to-Arranger) Ch. 7 Orchestral Free Bass
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Ok, so that is the basics, but what you can also do in more advanced setups is not just use the MIDI in or out... but the MIDI THRU (through) as well, and what that does, is become a pass-through for all 16 channels so you can do all kinds of interesting things... like, going OUT from an accordion, IN to the arranger, THRU that same arrange and IN to an external MIDI module, so what you could now in theory do is things like using an arranger for the drums and accompaniment and an external module for the right hand... or something more advanced like using an arranger for the drums and accompaniment and a specific instrument for the right hand and an external module for a secondary or layered instrument, again for the right hand... and on and on, more complex as you need them!
We've mentioned MIDI in out and thru, and the basic differences are:
MIDI IN... is the pathway in to a MIDI device, so it can hear a signal, but NOT pass it on to the next device in a chain of MIDI devices (cannot transmit)
MIDI OUT... sends the signals from the accordion outwards, but cannot hear any signals coming in.
MIDI THRU... is the pathway out from a MIDI device, so it can hear a signal, but CAN pass it on to the next device in a chain of MIDI devices (transmits to the next MIDI device)
Not all devices follow this standard, some have a MIDI out and thru that do the same thing, and that thing could be one or the other in terms of functionality and how it passes the received data forward (or not).
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Ultimately, the capabilities of MIDI are only limited by the complexity of the devices more than the technology. For example, bellows expression, key velocity or register/sound changes are signal functions that can be sent over any MIDI channel, and are *not* channel specific, like some think.
Oh, did you guys know that the creator of MIDI was also the creator of the V-accordion, indeed the creator of Roland itself? In June 1981, Roland founder Ikutaro Kakehashi proposed the idea of a method to standardize communication between digital instruments. Over the next two years, this standard was discussed and modified by representatives of companies such as Roland, Yamaha, Korg, Kawai, Oberheim, and Sequential Circuits and it was renamed Musical Instrument Digital Interface or MIDI. Dave Smith was another person that contributed greatly to MIDI's development.
MIDI's development was announced to the public by Robert Moog, in the October 1982 edition of Keyboard magazine and by the January 1983 Winter NAMM (or National Association of Music Merchants) Show. MIDI was demonstrated via connection between a Prophet 600 analog synthesizer and a Roland JP-6 keyboard synth. The MIDI Specification was published in August 1983 and implementation of MIDI within accordions followed very quickly. I myself have a 1983 Elkavox 83 that I bought new in November of that year with a basic MIDI already installed.
Since the MIDI standard was unveiled by Ikutaro Kakehashi and Dave Smith, both later received Technical Grammy Awards in 2013 for their key roles in the development of MIDI. Sady, Ikutaro Kakehashi died just very recently on April 1st, 2017.
Questions?