A loop back connector can be used to echo RS232 data transmitted by a serial port back into its own RS232 receiver. In this way, the function of the serial port can be tested. 

Simple Test - Paperclip test

The most basic loop back test is done by connecting the Transmitting pin (TXD) and the Receiving pin (RXD) of a serial port together. In this way data sent down the serial port will come straight back. When the functionality of the other data signals in not important simply short the TXD and RXD pins. For a 9 Pin or 25 Pin connector this means connecting pins 2 and 3. Alternatively if you have a jumper available this can easily to placed onto pin 2 and 3:

Loopback jumper over pin 2 and 3

Full Test

Follow the 2 diagrams below for a full loop back connector:

Schematic Representation

Schematic Representation of Loop-back connector for loop back test

Actual Representation

Physical Representation of Loop Back Connector


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