Configuring Communications Ports and Printer Ports

A communications resource is a physical or logical device that provides a single, asynchronous data stream. Serial ports, parallel ports, and modems are examples of communications resources. In Windows 95, VCOMM is the communications VxD that manages all access to communications devices. Port drivers use VCOMM to register themselves and to manage access to communications devices.

Two types of ports appear in Device Manager:

• Communications ports, also known as COM ports, serial ports, or RS-232 ports, are used to connect RS-232 – compatible serial devices such as modems and pointing devices to the computer.
• Printer ports, also known as LPT ports or parallel ports, are used to connect parallel devices such as printers to the computer. For more information about configuring printer ports, see Chapter 23, "Printing and Fonts."

Several types of communications ports might be listed in Device Manager:

• Serial ports, also known as RS-232 COM ports, to which external serial devices can be attached. These usually require a 9-pin or 25-pin plug. Serial ports designed for Windows 95 use the 16550A buffered UART, which has a 16-byte FIFO that gives the CPU more time to serve other processes and that can serve multiple characters in a single interrupt routine.
• An internal modem adapter. In addition to being installed in Device Manager, internal modems should also be installed and configured in the Modems option in Control Panel. For information, see Chapter 25, "Modems and Communications Tools."
• A PCMCIA modem card, if it is using PCMCIA socket drivers designed for Windows 3.1 instead of Windows 95-compatible PCMCIA drivers. In this situation, a PCMCIA modem card is treated as if it were an internal modem adapter; that is, it must be installed as both a COM port and a modem.

When you install a communications device, Windows 95 automatically assigns COM names to communication ports, internal modem adapters, and PCMCIA modem cards according to their base I/O port addresses as shown in the following list:

• COM1 at 3F8 (input/output range)
• COM2 at 2F8
• COM3 at 3E8
• COM4 at 2E8

If a device has a nonstandard base address, or if all four standard ports have been assigned to devices, Windows 95 automatically assigns the modem to COM5 port or higher. Some 16-bit Windows 3.1 applications might not be able to access ports higher than COM4. Consequently, in the Systems option in Control Panel, you must adjust the base address in Device Manager or delete other devices to free up a lower COM port.

In addition, if some of the devices installed on a computer are not Plug and Play-compliant, you might have to change resource settings for their communications ports. You can change communications port settings by using Device Manager, as described in "Installing New Devices" earlier in this chapter.

Tip

For future reference, you might want to record the settings that appear on the Resources sheet for each communications port.