Monoprice 101207 DB9/DB25 Serial Loopback Tester

The Monoprice 101207 DB9/DB25 Serial Loopback Tester is a simple but essential diagnostic tool for anyone who works with RS-232 serial interfaces. It functions by routing the transmit data line back to the receive line on the same port, allowing you to confirm that a serial connection can successfully send and receive data without needing a second device or cable run. This makes it invaluable for isolating whether a communication failure is caused by the port itself or by external factors such as cabling or the remote device.

The 2-in-1 design accommodates both DB9 and DB25 serial connectors, covering the two most common RS-232 form factors with a single piece of equipment. As a passive device, it requires no batteries or external power — you simply plug it into the port and run your preferred serial diagnostic or terminal software to send test data and verify the loopback. While it does not ship with testing software, most technicians already have access to utilities like PuTTY, HyperTerminal, or dedicated serial test applications. Its compact size makes it easy to store in a tool bag alongside other network and serial diagnostic equipment.