Listen32 is described as 'Tap into RS-232 transmit signals for monitoring bi-directional communications between two devices. Two COM ports are used in the listening device, the receive line of each connected to the transmit line of one of the communicating devices' and is an app. There are more than 10 alternatives to Listen32 for a variety of platforms, including Windows, Linux, Mac, Flathub and Flatpak apps. The best Listen32 alternative is CoolTerm, which is free. Other great apps like Listen32 are PortKiller, RealTerm, Tauno Monitor and YAT - Yet Another Terminal.