I have now dumped the firmware from the HP2640B terminal. Since the EA4900 ROMs are not much like normal EPROMs in regards to pinout and supply voltages I resorted to use a logic analyzer to dump the contents of the ROM simply by parsing out the ROM accesses while the terminal did the selftest. It took a quite many dumps to get the full firmware out of it since my analyzer HP16550 boards have limited storage. Someone that has a HP16555A board to sell?
http://storage.datormuseum.se/u/96935524/Datormusuem/HP2640B/2640.hex One file for all four ROMs. The good thing attaching the analyzer was that it helped me to find the two bad 2102 chips on one of the memory boards. Now I get the full 5kKbytes. While going through the HP2640B I found two boards which I cannot find relevant schematic and description for, the 02640-60009 Display Memory Access Module and the 02640-60088 Display Timing Module. Anyone has more info on these? With the firmware downloaded I tried to investigate why the small Diagnostic Download example described here: http://computermuseum.informatik.uni-stuttgart.de/dev_en/hp2644/diag.html The example tries to call address 315 octal, but the firmware I dumped has a HLT instruction at this location which explains why the terminal hangs completely. The HP2644 firmware is most likely different from the HP2640B firmware. http://www.datormuseum.se/peripherals/terminals/hp-264x