What is a Loopback Test?

Kurt Inman

A loopback test is a simple method to determine whether communication circuits are functioning at a basic level. It often involves connecting a loopback device to a circuit and verifying that transmitted data returns to the sender. Loopback testing is commonly performed on serial, network and telephone links. Technicians use this method to quickly test whether data ports, cables and connections are working.

Cat 5 cable with RJ45 plug.
Cat 5 cable with RJ45 plug.

Performing a loopback test is generally a multi-step process. For example, a loopback device may be attached directly to a computer's serial port. If data is sent and then received on that same port successfully, it is likely that it is functioning correctly. The loopback device can then be moved to the end of a cable attached to the port, and data can be sent again. If that passes, the cable is probably working correctly, and the device can be moved further along the link for more testing.

A loopback test can be used to test the ports on a router.
A loopback test can be used to test the ports on a router.

The loopback device can be as simple as a wire routing the transmit-data line to the receive-data line. The simplest form of a Recommended Standard (RS) 232 serial loopback test can be done this way using a standard serial connector. A more thorough test can be performed by adding a few hardware handshaking signals to the connection. For example, the Clear To Send (CTS) line can be wired to the Request To Send (RTS) line. The Data Terminal Ready (DTR) line should also be wired to the Data Set Ready (DSR) line.

RS 422 serial connections were used on AppleĀ® computers prior to the adoption of the Universal Serial Bus (USB). RS 422 and RS 485 use a pair of differential signals for transmit-data and another pair for receive-data. The CTS and RTS hardware handshaking signals are also paired in this manner. Each of these pairs must be connected to their corresponding pairs for a loopback test. For example, the positive transmit-data line connects to the positive receive-data line and the negative to negative.

An Ethernet loopback test can be performed with a similar connection of data lines. The positive transmit-data line is wired to the positive receive-data line. The negative data lines are routed in the same way. These connections are often wired on a customized Registered Jack 45 (RJ45) jack or port. Other network standards such as Bell Labs T-carrier 1 (T1) and Gigabit Ethernet can be tested with a similar cabling.

More complex network loopback testers are also available that are more like stripped-down protocol analyzers. In addition to the basic loopback test, this type of analyzer also checks signal quality, data loss and similar characteristics. Many Information Technology (IT) workers use these devices in the daily monitoring of their networks.

Ports for the Recommended Standard 232 (RS-232) cable are still found on many personal computers.
Ports for the Recommended Standard 232 (RS-232) cable are still found on many personal computers.

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