wow。。so cool !!!! serach SX24M8 on taobao ,you will get a 35RMB logic analyzer on USB2.0 ,up to 24M sample rate ,,,maybe it enough for 991+..... 退坑很久的我。。。(((
i have logic analyzer,arduino and 991+..,mabye not necessary for arduino,the analyzer software let you gather signal and can be save on your computer directly.. your research really interesting...... i havent play with calculator since 2013.... almost forget most of these....maybe you can do it yourself,all you need can easily get on taobao with very low price....
Now I've switched to reading the ROM from LCD screen, thus don't need special hardware. And I had succeed in getting the ROM. Thanks for anyone (yls_1996 only) who had offered to help. I may upload the ROM here for anyone who wants to take a look at it.