🚀 go-pugleaf

I have a vt220 by digital, the real thing, not a clone. What is the
'Compose Character' key for? How can I use it? Can I 'save' stuff to the
function keys so that I power up with Hayes AT commands on them? Are there
any roms out there for Japanese characters sets? More generally, would 
some kind soul send me some information about the differences between the
various vt### terminals? I always use the vt100 set up, how would vt220
be better? Can graphics be viewed, like gifs or bitmaps?
Any responses would be apreciated. I would esp. like encyclopedic amounts
of information :-)