The details of these depend on your actual PC installation, but some properties of it can be described independently from what exactly you have.įor example, the question whether you have two native COM ports or one native plus another one that is emulated on a USB port is not relevant to the question what happens on the serial line actually (because it is the job of the adapter that the other side is compatible with native COM ports). Your question suggests that you are interested in what is going on beyond the serial connection itself, so please continue reading below.įrom the keyboard to the screen, there are various (hardware and) software components involved. The connection between the two COM ports (of your PC and the adapter at your PC) forms the core part of the loopback test arrangement.

The important feature why such a test works easily on the serial port of your example is that the interface is so low-level that there is no connection layer (or hardware feature) in between on which the receiver side could notice that the signals aren't posted by an external peer host, but that they come from the local host. With only little abstraction, the logical loopback you build from one COM port of a PC to another COM port at the same PC has in common the feature that you can see your own transmission being returned at the same display screen. You could also perform a similar (yet more simplistic) test using a minimal configuration where the loopback is just a directīetween the TxD/RxD lines of the very same COM port. To the Received Data line (RxD) of a port at the same host. Signal lines of the RS-232, the Transmitted Data line (TxD) is The COM ports you connect are the typical PC implementation of the RS-232 serial interface.Ī Loopback configuration is just what the name suggests:

The question is pretty wide, but I give it a try:

Loopback license free