A friend and I marketed a replacement for the MDA card on a 5150/5160 that used a Z80 and a bunch of parts to allow the PC user to replace their display card and hook up a VT-100/220 style terminal--and switch or split-screen the PC data with other serial data (say, from a minicomputer) on the terminal. It also took the terminal keyboard codes and translated them to 5150 scan codes. It had a certain appeal, but wasn't fast enough to work effectively with a 5170 and would not do graphics. I still have the board (and a few abandoned prototypes).