Found it... Yikes!
Personally, external control of arrangers is one of my pet peeves. Who wouldn't like to be easily able to link ANY two arrangers together, and have them work as one?
Quite honestly, two MOTL (or even BOTL) arrangers from two different manufacturers, when combined, is usually better than the single TOTL arranger from either! You get a much wider palette of sounds, and can use each of their strengths to counter the other's weaknesses.
But do the manufacturers make this possible? Hell NO!
Utterly incompatible control codes, with no proviso to set them to what we would prefer. Sure, you can link two Roland's, or two Korg's, and they will work... but who in their right minds wants THAT? I want to link a PSR with a G70. Or a PA3x with a BK-7m. Or a BK5 and a PA600... Now THAT would be useful!
Please, please, please... contact your arranger manufacturer, and pressure them into a standardized set of codes for basic arranger operation. We don't need the MIDI Standards Committee to issue new codes (we'd be waiting until NEXT century!). We can use the codes that already WORK.
My suggestion would be the system that uses PC/32/00 codes, that Korg and Roland use (although theirs are different, LOL) and possibly others (don't know Ketron's codes). This, of course, would make mighty Yamaha have to step back from decades of inertia, but c'mon! Sys-ex for Division selection? WTF!
Basically, in practical terms, all you would need is codes for Variation and Fill selection, the Break (of whatever kind it is), Intro and Ending codes, and some kind of communication about transpose selection and if you want to get fancy, OTS selection (optional, IMO).
Tempo is handled by the Master, so rits and accels and initial tempo is good to go.
You know, this is not an awful lot... about the only thing not currently coded is the transposition parameter. The rest is simply having the guts to standardize.
It makes SO MUCH sense for the manufacturers to do this. I can tell you straight away... if I could run a PSR from my G70, or a BK-7m from a PA600 or Tyros4, I'd already have bought those! And I'm sure a LOT of us would. Every arranger has strong points. And every arranger has weak points. But put two together, and one covers the others' weak points.
Only thing holding it up is reluctance by the manufacturers to get together and standardize. But I remind you all... if it weren't for a few manufacturers getting together and going 'It's insane for us all to have exclusive, proprietary ways of communication!' back in the early eighties, we wouldn't have MIDI, and the synth explosion would never have happened.
There is MONEY to be made! Let's go!
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An arranger is just a tool. What matters is what you build with it..!