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TABLE 7.1 COM Port IRQ and Default I/O Addresses


Port


IRQ


I/O


COM 1


4


3F8


COM 2


3


2F8


COM 3


4


3E8


COM 4


3


2E8

Configuring an External Modem


Although almost all modems used today are internal, there are some situations when external modems may be more appropriate (such as modem pools). Also, some people prefer external modems because they can see the modem’s status lights. Most internal modems use software status lights, which don’t work if the hardware or software is failing. When using external modems, you have two considerations: available serial ports and the UART type.

Available Serial Ports


With the large number of external serial expansion devices available— including modems, cameras, and printers—a spare serial port is often not available, and so you have to purchase an internal modem. A technology called the Universal Serial Bus (USB) is now available on almost all new computers. The USB allows up to 127 devices (including modems) to be chained off of a single serial port. This technology will eventually eliminate the current need for multiple serial ports on a computer.

UART Type


UART stands for Universal Asynchronous Receiver-Transmitter and is the chip that manages serial communications. Each set of serial ports shares a UART. The type of UART chip determines the maximum port speed that a particular serial port can handle. There are two main types: the 8250 and 16550 series. The primary difference between them is the capacity of the port buffers. Internal modems have built-in, high-speed UART chips, so this isn’t an issue with internal modems.

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