However it will be able to test the basic functionality and provide a good level of confidence about the ability of the equipment to send and receive and generally check the operation of the serial port. The test loopback connector approach to testing has its limitations as it will not be able to detect speed mismatches or the like as the same issue is likely to be present on transmit and receive.
In this way the equipment can be used on its own to check that it can communicate, even if it is only with itself. When testing RS232 links, it is often useful to have a test loopback connector. Serial Data standards: Serial data standards RS422 RS449 RS485 20 mA current loop RS232 basics RS232 standard Software handshaking Signals & voltage levels Pinouts & connectors Loopback connections RS232 cables RS232 Loopback Connections: test connector DB9 & DB25 D-type connector pin connections for an RS232 serial loopback test.