Just some new facts, I recently found. In case one is interested or one stumbled just here.
That PLC for my programmer was called FPC 606 and introduced at Hanover Fair in 1980. The first PLC from Festo and one of the world’s first multitasking PLCs (64 tasks). Electronics first developed since 1976.
The 606 isn’t mentioned in the manual nor on PLC cards. Just Festo FPC. Not sure if they changed the name, maybe with the next model 404 (1984). Maybe mine (my manual) is a prototype. Sample ROM date in code is 1979. The only larger official photo I found on a 1995 Hungarian Festo newsletter. I later found a sticker with 606 on a photo on the back of an empty rack.
FPC 606 on back here with stand-alone rack (cage). On left a 404 (compact version) and the Programmer PRG-32 with membrane keyboard, either for 404 or both. It has 8 instead of 9 verticals keys and a long 4-line alphanumeric LCD.
The programming of the 404 is similar. That has an additional STEP command but same IF/THEN/OTHERWISE instructions. Software now for MS-DOS. So completely different opcodes and multiple files with headers to be compiled.
Interesting that the 2 left keys (otherwise, furthermore) on mine counting as release keys.
The last thread is this (with help from an expert) start address found at 400 with ROM pairs 8-4 7-3
Homebrew mini metal PDP-8 (1976) Intersil 6100 code
Also interesting this thread in where I found the mnemonics