A null-modem cable is used to allow two data terminal equipment (DTE) devices such as computers to communicate through RS232 interfaces without requiring a modem or other type of data circuit-terminating equipment (DCE). A null-modem connection enables two computers to transfer data directly without using a network or removable storage media.
A Virtual Null Modem is a software tool which can emulate RS232 COM ports that are connected with a virtual null-modem cable. It also enables you to create custom-named virtual serial ports that you can connect with a virtual null-modem cable. The application allows you to create as many virtual ports as you want, limited only by your system resources.
Two programs that communicate with serial ports need to use a null-modem connection. Typically this is done with a physical null-modem which is a cable that directly connects the serial ports. This implementation requires two computers or a single machine with two physical COM interfaces. Using the Virtual Null Modem utility, you can simply emulate the physical connection and run both programs on the same computer without real ports or any extra hardware.
There are a number of null-modem emulation software tools available. Com0com is a freeware, open-source null-modem emulator application. It is used to create a serial interface for device emulation programs. What this means is that it provides a pair of serial ports, one of which connects to the device emulation program while the other is employed by the Windows application that wants to communicate with the device.
Virtual COM Port Driver is a commercial virtual null-modem solution developed by Eltima Software. It is a professional-grade utility that creates pairs of virtual COM ports that can be connected with a virtual null-modem. The virtual port pairs provide a communication bridge enabling data transmitted from an app at one end of the pair to be received immediately at the other end. It’s a feature-rich solution to problems caused by the lack of physical serial interfaces on modern computers.
There are both advantages and disadvantages to using paid or freeware software solutions to address serial connectivity issues. To help you decide which is better for your situation, we present this table that looks at the features of Virtual Serial Port Driver and Null-Modem Emulator (com0com).