Before you buy a serial port, verify that you don’t already have what you need. Serial ports are cheap, ubiquitous, and easily disabled, so you may already have one or more of them installed that you don’t know about. Before you purchase a serial port expansion card, take the following steps:
Use the operating system or a diagnostics program to determine the number and configuration of installed serial ports.
For Windows NT 4.0, choose Start → Programs → Administrative Tools (Common) → Windows NT Diagnostics, click the Resources tab, and double-click the Serial item.
For Windows 2000/XP, right-click the My Computer icon and choose Properties. Click the Hardware tab and then the Device Manager button. Locate the Ports item in the Device Manager tree and double-click it to expand the listing to show individual ports. All installed COM ports (and LPT ports) are displayed. Double-click a port to display a tabbed dialog that provides information about that port, including details about General configuration, Port Settings, Driver details, and the Resources that port is using.
For Windows 9X, right-click the My Computer icon, click the Device Manager tab, expand the Ports (COM and LPT) item, highlight a port, click Properties, and finally click the Resources tab. Note the COM port number, IRQ, and I/O address assignments for each port present.
For Linux, open a terminal window. Display /proc/devices and look for tty* devices in the Character ...