You can have near-end loopback, where you make the loop at the outbound interface on a device, and far-end loopback, where you make the loop at the distant end of a circuit or at progressive points on the way to the distant end. This enables you to do a simple bit by bit test that you're receiving what you send, or calculate statistics like bit error rate on the raw physical medium. It means, usually physically, looping the communications medium back on itself so that anything you send out as a transmission comes immediately back to you, unaltered, as a receive signal. And of the responders, only mikeselectricstuff seems to understand what "loopback" means. I suppose some USB PHYs may have some sort of loopback-like test mode but I've never looked for or noticed one. USB being having an intrinsically master/slave relationship between hosts and devices it isn't really possible to do a meaningful loopback test. Well you could try opening the data sheet for the LAN8742 (presuming that is the PHY chip you're using, you mention it but you don't actually say) search for the word "loopback" and you'll find both the control register bits to enable near and far end loopback modes, and descriptions.
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