Ok one last post. Had my network gurus out. They brought an ancient 10/100 hub. Success!
IP settings as follows:
PC: 10.50.50.2, mask: 255.255.255.0 No gateway
ATEM switch (rack unit):10.50.50.5 mask:255.255.255.0, gateway:10.50.50.2 (yes I know this is strange, but it was the only way it worked)
Looks like my gig switch is dead or incompatible, so I’ll replace it. Also tried both a regular ethernet cable and a crossover cable on a mac and a pc and all fail.
For what it’s worth…