> What am I missing?
Not much, but consider latency: You can use the Group Execute Trigger (GET) to simultaneously trigger multiple instruments with both very low latency and very low latency dispersion. Think, easy-to-use sub-microsecond synchronization.
Ethernet and USB 4 may have orders of magnitude more bandwidth, but can’t achieve the same multi-device synchronization capability without side channel signals.
Now, sure, you can add the same capability with a programmable pulse generator connected via coax to the trigger input of all your instruments, but GBIP lets you do that with just the data connection (and you don’t always have a spare trigger channel). The only other protocols I know of with similar capabilities are PXI and PXIe, which are “PCI(express) in an incompatible form-factor, plus some extra signals for real time synchronization”.
sub-microsecond triggering should be doable with a level-2 cut-through switch and an ethernet broadcast no? I admit that ethernet is not really designed for that as the Phy is then the latency bottleneck.
Sure, in principle, but that takes effort and special equipment to set up. The point is that GPIB makes it easy (trivial, actually) with nothing more than the cables you normally use to connect instruments to get very low and predictable latency.
GPIB GET works by first configuring a subset of bus devices as listeners and then sending a single-byte message (it’s an 8 bit bus, so one bus cycle) with the ATN line asserted. It’s intrinsically low latency without any special effort.
Whether that makes it worthwhile to put GPIB on a new instrument in 2025 is a different question. I’m only addressing “what does GPIB give you”?
There are approaches to real-time ethernet (some industry implementations like profinet or ethercat, 802.1as from IEEE) but support is spotty and it requires specialized gear to be effective.
If you buy used equipment which doesn't have Ethernet or your company wants you to ise the stuff that is in the Lab since 10+ years there's simply no other choice. Or companies that see Ethernet as a potential security attack vector.
It's indeed not that GPIB is better than Ethernet. In tiny aspects that's argueable, but as general statement true.
> Or companies that see Ethernet as a potential security attack vector.
It's sad how true this is. VXI gets corporate IT all prickly. An airgapped lab network would be safer but somehow they hate that idea even more.
GPIB isn't even on their radar. Test to your heart's content. It's not a "network".
Nothing, other than perhaps just much high-quality legacy equipment is out in the field that still works fantastically.