![]() PDH) and similar on standard coax patch panels there were/are 'U-links' made specifically to allow the adjacent tx and rx connectors to be joined to form a far-end loopback. Loopback plug For telecoms E1 links ( i.e. (Gigabit ethernet gets more involved at the signalling level and switches have OAM modes to provide loopback, not quite as satisfactory as literally testing just the wires in the way a classic physical loopback does.) Most Ethernet PHYs will have a test mode where you can command the PHY to cross connect its tx and rx paths just before they leave the chip. For 10/100Base-T Ethernet on RJ45s you can wire a plug to cross the receive and transmit pairs. ![]() Sometimes referred to as hairpining* because a crude way of doing it was to borrow a hairpin from a secretary and physically stuff the ends of it into a comms device's socket's tx and rx pins - not recommended. pinging) as you have no way of knowing if they are operating correctly. The whole point is to enable one to test the physical connection without getting any other systems involved (e.g. 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. ![]() Fiber optic loopback testing is the easiest way to ensure that the transceiver works faultlessly.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. Then we can compare the transmitted pattern with the received pattern to make sure they are identical and have no errors. Thus, during the testing process, the loopback module directly routes the laser signal from the transmitter port back to the receiver port. ![]() The former one is to send out laser signals and the latter is to receive signals. As we all know, fiber optic transceiver has two ports, a transmitter port and a receiver port. And the fiber loopback test can be utilized to check whether the fiber optic transceiver is working perfectly as designed. ![]() It is used as an aid in debugging physical connection problems. But here I’d like to introduce one of its common application-fiber optic loopback testing.Ī fiber loopback test is a hardware or software method which feeds a received signal or data back to the sender. Fiber optic loopbacks can be used in a number of applications such as equipment interconnection, device pig-tailing, premise networks, patch panel applications and communications connections. ![]()
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