Hello,
my first post.

I'm trying to find a midi interface that can do realtime processing in patch mode.
In addition to usual merging and patching, I'd like to filter, convert, transpose, split etc.

Also, if there exists an older serial port machine that can do this, please let me know.

It's not exactly "stand-alone", but of course I can always get a laptop running midi pipe
and be happy with one of my Unitor 8's.

Best,
Petja

The rack module for the Korg Z3 guitar synth has something like what you are describing here. You can use it with the guitar pickup, which is surprisingly good for its age, and can control bank and program values via the FC-6 external (foot-stompers) controller. It features a real-time patch-mode for live play, and I believe it has a serial port in addition to the usual MIDI in/out/through. It was, incidentally, the product of an odd relationship with YAMAHA, whose design it allegedly was. Interesting set of boxes. Have a look at the pickup, which I think was designated ZD-3.

More later when I get mine back.