Loopback testing is a method of checking communication circuits by connecting a loopback device and verifying that the transmitted data returns to the sender. It is commonly used for serial, network, and telephone links. The loopback device can be a simple cable or a more complex analyzer that checks for signal quality and data loss. Technicians use this method to quickly check if data ports, cables, and connections are working.
A loopback test is a simple method of determining if communication circuits are functioning at a basic level. It often involves connecting a loopback device to a circuit and verifying that the transmitted data returns to the sender. Loopback testing is commonly performed on serial, network, and telephone links. Technicians use this method to quickly check if data ports, cables, and connections are working.
Running a loopback test is usually a multi-step process. For example, a loopback device can be connected directly to a computer’s serial port. If data is successfully sent and then received on the same port, it is likely to work fine. The loopback device can then be moved to the end of a cable connected to the port and the data can be sent again. If that passes, the cable is probably fine and the device can be moved further along the link for further testing.
The loopback device can be as simple as a cable that routes the data transmit line to the data receive line. The simplest form of a standard recommended (RS) 232 serial loopback test can be performed in this manner using a standard serial connector. A more thorough test can be done by adding some hardware handshake signals to the connection. For example, the Clear To Send (CTS) line can be connected to the Request To Send (RTS) line. The Data Terminal Ready (DTR) line must also be connected to the Data Set Ready (DSR) line.
RS 422 serial connections were used on Apple® computers before the adoption of the Universal Serial Bus (USB). RS 422 and RS 485 use one pair of differential signals for data transmission and another pair for data reception. The CTS and RTS hardware handshake signals are also coupled in this way. Each of these pairs must be connected to the corresponding pairs for a loopback test. For example, the positive data transmit line connects to the positive data receive line, and the negative one connects to the negative one.
An Ethernet loopback test can be performed with a similar connection of data lines. The positive data transmission line is connected to the positive data reception line. Negative data lines are routed the same way. These connections are often wired to a custom Registered Jack 45 (RJ45) jack or port. Other networking standards such as Bell Labs T-carrier 1 (T1) and Gigabit Ethernet can be tested with similar cabling.
There are also more complex network loopback testers available that are more like scaled-down protocol analyzers. In addition to the basic loopback test, this type of analyzer also checks for signal quality, data loss, and similar characteristics. Many Information Technology (IT) workers use these devices in day-to-day monitoring of their networks.
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