3C5X9X.EXE should be a self-extracting archive. Wonder why it would check for the existence of a card...
I've attached the config utitility and packet driver i'm using on my 286 machine, maybe it works for you.

(There seem to be different versions of the driver package for 3c9x5 with so-so compatibility, maybe it's the same for the 3c509 as well)

EDIT: Also take a look at https://github.com/hackerb9/3C509B-nestor

Sorry, I meant that when I try to run the installation after it extracts I get that error. The extraction itself has no issues.

I tried running the file you shared here (thank you) but it had the same error. I also checked out the github link you shared. I burned those files onto a cd and tried it, i thought perhaps PNPDSABL.EXE would be helpful but alas, the same error occured with that and every other .exe in the folder:

Apparently only possible (officially) with an EISA rig or by 3Com themselves...unofficially there are ways, though not all exactly easy / guaranteed

by Hardware Reset

by Linux 3Com Tool Recompile

http://krusty.dt.e-technik.tu-dortmund.de/~ma/3c509.html

You may have to hunt around online or on the Archive for alternate sources of some of the software files linked

Thank you guys for the help. I ended up ordering another of the same card, which was thankfully configured for ISA and everything seems to have gone well. (Not entirely sure how to test it out yet but I'll figure it out).

Dunno what I'll do with the EISA one for now, maybe I'll build an EISA computer some time in the future. So strange that it's that difficult to reconfigure.

MetroidHatchling wrote on 2021-04-26, 20:18:

Thank you guys for the help. I ended up ordering another of the same card, which was thankfully configured for ISA and everything seems to have gone well. (Not entirely sure how to test it out yet but I'll figure it out).

Dunno what I'll do with the EISA one for now, maybe I'll build an EISA computer some time in the future. So strange that it's that difficult to reconfigure.

That's a downside of the EISA design. EISA mainboards free up extra port address ranges that are occupied by traditional ISA mainboards. There are obviously no ISA cards that use these port addresses, because they would be in conflict with most ISA mainboards. Focussing EISA cards onto these extra resources makes port conflicts between them and ISA cards nearly impossible, which is mostly a good thing. On the other hand, you can not talk to an ISA card using the EISA protocol in a non-EISA mainboard, because the conflicts that kept the EISA ports free will occur in a classic ISA board.

If you are lucky, the conflicts are not bad enough to prevent a software tool to switch the 3c509 card out of EISA mode, but this is operating the mainboard out-of-spec, which is why 3COM chose to not attempt to that, but some 3rd party tools linked above do so.

MetroidHatchling wrote on 2021-04-26, 20:18:

Thank you guys for the help. I ended up ordering another of the same card, which was thankfully configured for ISA and everything seems to have gone well. (Not entirely sure how to test it out yet but I'll figure it out).

Dunno what I'll do with the EISA one for now, maybe I'll build an EISA computer some time in the future. So strange that it's that difficult to reconfigure.

Now you've got a working replacement, it'd still be worth trying to force the original card out of EISA mode. Here's a post from someone who fixed the problem, albeit down to a different reason

Re: ISA 3Com EtherLink III 3C509B-TPO Repeatedly fails self test, "Vital EEPROM," error.