A null modem is a cable or adapter that allows communication between two serial devices without a modem. It is used for debugging operating systems and accessing devices without a built-in console. A crossover cable is a type of null modem where serial data lines are crossed within the cable. Hardware handshake signals can also be wired into a null modem. Before USB, a null modem was used to share files between two computers. A common modern use is to access an operating system or non-console device with a separate computer.
A null modem is a cable or adapter that allows two serial devices to communicate without a modem. Some null modems only connect data lines between devices, while others include handshake signals as well. Null modems are often used by engineers or technicians to communicate with devices that don’t have a built-in console. These include many types of embedded systems, network switches and servers. They are also often used for debugging operating systems when no keyboard or video service is available.
A crossover cable is a null modem connection where serial data lines are crossed within the cable. The Transmit-Data signal on one end of the cable is connected to the Receive-Data pin on the other and vice versa. This cross connection can also be implemented in a null modem adapter. In this case, a standard serial cable connects a device with the adapter. An identical cable connects the adapter to the other device, and the crossover is wired inside the adapter itself.
Serial hardware handshake signals can also be wired into an adapter or crossover cable. The Request-To-Send line of one device can be connected to the Clear-To-Send line of the other and vice versa. Depending on the hardware and software on both sides of the connection, this may be sufficient for communication to continue.
Three additional hardware handshake signals can be implemented in a null modem. Some serial devices require Data-Terminal-Ready, Data-Set-Ready, and Carrier-Detect signals to be functional. When all three of these signals are asserted, each device recognizes that the other is on and connected. Many null modems connect Data-Terminal-Ready on one end to Carrier-Detect and Data-Set-Ready on the other. However, some null modems may not connect Carrier-Detect signal.
Before the development of the Universal Serial Bus (USB), a null modem was often used to share files between two computers. A direct cable connection was made between the two systems, using their serial ports and a null modem. File transfer software designed for a modem link was then used to copy files from one system to another. This was especially useful for very large files that could not fit on a floppy disk or other removable storage media. For the same reason, a direct connection was commonly used to transfer large numbers of files in one batch.
A common modern use is to access an operating system (OS) kernel or non-console device with a separate computer. Often when debugging an operating system kernel or diagnosing a kernel panic, the standard keyboard and display cannot be used. A null modem can give a technician control of the system during the debugging session. Many devices are designed to operate console-free, such as network switches, blade servers, and embedded systems. When a keyboard and display are temporarily needed for diagnostics or maintenance, a null modem is a common means of access.
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