What are loopback cables?

By AJ Born | June 04, 2024

What are loopback cables?

Meet the Connector: Loopback Cables 

A loopback cable is a diagnostic tool for testing the functionality of a physical port in networking systems. They are used to assess and resolve any issues with a network interface or network interface card (NIC) without linking to an external device. The loopback cable simulates a network connection. It is plugged into a port (such as a serial, parallel USB, and network port) to perform a loopback test. It redirects outgoing electronic signals and digital data streams back to its source as input data.

Telecommunications, serial interfaces, virtual network interfaces, and networking all implement loopback techniques.

Loopback cables, also known as loopback plugs or loopback adapters, are typically used for fiber optic testing applications or network restorations. There are versions for many different ports, including serial ports, Ethernet ports, and WAN connections. The connectors on the loopback cable correspond to the interface being tested.

BizLink QSFP-DD loopback module tests QSFP-DD ports by looping back electrical signal. This module is an 8-channel transmission which supports data rates of up to 400 Gb/s. QSFP-DD loopback are backwards plug compatible with existing 100G based systems and support 200G (8 channels @ 25G NRZ) or 400G (8 channels @ 50G PAM 4) signaling transmission.


Samtec’s FireflyTM loopback cable assembly for testing the Firefly connector system operates at data rates of 28 GB/s and higher and utilizes 34 AWG, 100 Ω ultra-low skew twinax cables to support high-speed differential pair paths. Outbound TX signals are routed back to their corresponding RX pins.


Rosenberger’s fiber optic loopback adapters feature ruggedized housing equipped with latches for secure fastening for use in harsh environments and are tested to work in extreme temperature ranges. In addition to testing the transmission capability and receiver sensitivity of fiber optic network equipment, they may be used for the burn-in stage of boards and other equipment.

Design Notes 

Connector types: Connectors are the same on both ends, such as RJ45 for Ethernet or DB9 for serial connections. The connector types are chosen based on what is being tested.

Gender: Classified as male or female.

Markets and Applications 

Datacom, Telecommunications

Telecommunications, serial interfaces, virtual network interfaces, fiber optic networks


BizLink, Samtec Inc., Molex, Rosenberger, Amphenol Communications Solutions

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AJ Born
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