You need to get a BTR. (Behind the tape reader)
I believe your machine in Pre RS-232. So there is no "conversion" to RS-232
A BTR will trick the tape reader into thinking it's receiving punched paper tape when it is receiving binary code from the PC.
In other words: The PC sends out Binary code (1001001), The BTR converts the binary code to a paper punched signal. The control accepts the code.
Some BTR's only SEND and do not receive programs back to the PC. That's OK though because editing on the older controls is not userfriendly. You just have to do you editing/debugging on the PC and send the program back o the control when you are done.
My Old SL3B has the YASNAC 2000G control. This setup works well.
It is not easy to get it set up. You MUST have a BTR that will work with the FANUC 6. You must have the SPECIAL WIRED CABLE. An off the shelf cable will not work. And you need the communication software for the PC to interface with the control.
Google "BTR for fanuc 6" To find some of the companies that make the package I have discussed. It's not cheap. I had my local retired Mori tech make me the BTR board and sell me his software to get passed this problem.
If you want his info I can give it to you.