Do you have another machine to test in? Then you can rule out the adapter.
Also, it probably needs a bidirectional parallel port. Are you using one?
Also make sure the 2nd LPT port is not conflicting with the Tandy one.

I actually did test in another machine, it seems to be fine, using the same parallel port card.

Something in the Tandy must be interfering with the expansion card.

I've configured it for the LPT2 default settings but it's not even recognized by norton sysinfo, so something's DEFINITELY a bit wonky.

I would suggest checking out TVdogs archive on Tandy stuff, I think Trixter also has an archive he hosts (or maybe that links to TVdogs site, not sure). You may be able to find manuals online on archive.org.

keenerb wrote on 2017-01-08, 02:44:

Has anyone successfully installed a Xircom pocket ethernet adapter on a Tandy 1000 series (non-RL)?

Using a third-party parallel port card I can get the adapter recognized but I receive an error "Pocket Ethernet III Failed Loopback Test".

I'm not sure if the pocket ethernet adapter has failed or I'm doing something wrong...

I happen to be having the same issue, granted on an IBM ThinkPad 360. Did you ever find the problem?

The problem fixed itself at some point during my tinkering.