You should be able to reload your parameters from your PC if you manually enter a few parameters for the serial port. For example:
Parameter 002, bit 5 (the 6th bit from the right) Turns on the serial port option
Parameter 003, bit 3 (the 4th bit from the right) Turns on the ISO code option
Parameter 311 (sets the baudrate, stop-bits, and handshake method for serial port)
Parameter 340 (input device)
Parameter 341 (output device)
Normally I set parameter 311 to 11001001. That's for 4800 baud, 1 stop-bit, and Xon/Xoff handshaking. These parameters can be added with the E-stop button pushed in.
On the SETTING page, set the PUNCH CODE to "1" for ISO, then set INPUT DEVICE 2 to a "1". You may have to release E-stop to change this setting. You'll need to be in MDI mode, display the SETTING page, then cursor to the item you want to change, press "P" then either "1" or "0", then INPUT.
With those settings you should be able to download your parameters from the PC using some DNC software.
To read in the parameter data, get your DNC software ready to send the file then, with the E-stop button pressed, turn on the Parameter/Enable switch on the main PCB, press the PARAM key to display parameters, then press the letter "P", then "-9999" then READ.
The Fanuc system 6 controls have EPROM chips for the main executive software, and some additional EPROMs are added for certain options. The ROM parity alarm you got was indicating that one or more of the EPROMs was either bad or missing. This can happen if you try to put an option parameter into the CNCs memory when the matching EPROMs for that option are not installed, or if one of the EPROMs is coming loose in its socket. The 6T-A control had several rows of EPROMs on the main board, but the 6T-B control has a separate "ROM" board with all the EPROM chips. Whichever one you have, be sure that they're all well seated in their sockets. Sometimes, after many years of hot/cold cycles, the EPROMs can actually back out of their sockets. Applying firm thumb pressure to each chip to be sure it's seated would be a good idea.
If you put all the parameters back in and get the ROM parity alarm again, see if it gives you any numbers. Usually the system 6 displays the address of the offending EPROM chips.