Yes, the display has a BCD encoding (forming one word) but it’s very complicated. 4 Locations, each with a different address, one has 4 digits and can just display octal values. 3 can display decimal and/or octal values, 2 of them can display special characters and 2 other ones work together. There is a table in the manual ,but I assume it’s not stored as table in the ROM.
E9 7777
*F9 999
I also mentioned that on my other thread
Help reading EPROM (Intersil IM6654A) and analyze firmware - #4 by mainframetom
A new fact is that the 3 digit decimal position in fact holds a value of 4 digits.
I can only read and write to the display of the attached PLC CPU card (which has the same style) so at least the addresses are different.
But I’m mainly searching for the mnemonics, being printed out. If I found these, I will be sure to have the correct ROM merging and then also the right PDP code.
I think, I even have found the location at 2000o the 3 character words are surrounded by @, but the 3 characters are wrong. DIV sounds plausible but isn’t a mnemonic. The @ seems correct.
So either another encoding or even encryption. I’ll now try to write the c code for the 2/3 encoding. I would need a program, searching for text while scrambling bits and bytes in all possible combinations.