I discovered that MAME have some DEC VT emulations. At my college life, we used VT102, VT125, VT220, VT240 and GIGI terminals. Most emulations in MAME are not working at this time. Only VT240 emulation is working well. I noticed a few VT emulations like VT330/VT340 had not implemented yet.
I discussed with some SIMH users on SIMH mailing list about graphics terminals. Someone mentioned about VT340 firmware dumps. I have complete dumps here. Check them in https://avitech.com.au/?page_id=1964
for more information. You can get dumps from website. Go ahead to implement VT340 emulation into MAME source that I need to access ReGIS/SIXEL graphics access from mainframe emulators. Only VT240 provides graphics access at ReGIS commands. I want to see colors in VT241 and VT340.