I have seen that post, however never worked with disassembler in any way, nor patched an DOS MZ executable. Your reply was more clear now, thanks for explanation.
What can go possibly wrong to at least try, right?
CEC PC-488 have factory default starting address
0x2B8, used range is
2B8-2BF according to manual.
After watching some tutorials, managed to patch exe in RAW mode using ghidra. Executable must be opened in RAW mode so it can be exported back to DOS MZ executable correctly.
After a few HW configuration trials, some patches/exports, finally managed to get instrument detected

Btw, couln't find PC2A refference around offset 0x346b2, is the offset correct??
Few notes:
- PC2A card is selected in startup batch
- CEC PC-488 *MUST* have IRQ enabled in my case (IRQ7)
- I'm getting "Spurious SQR error" which so far have no impact to functionality
I'm not sure if it's allowed to attach TEK FAS executables, so I'll share it upon request
