UPGRADING REPAIRING PCs

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Chapter 6—Serial Ports and Modems
Male and Female, 25-pin
Parallel loopback connectors
Figure 6.3 Typical wrap plugs including 25-pin, 9-pin serial, and 25-pin
parallel versions.
Modems
Modems provide a vital communications link between millions of
small- to medium-sized businesses and homes and the Internet,
electronic banking, and other services. The following information
will help you get the most out of your modem.
Modems and Serial Ports
External modems connect to existing serial ports and don’t contain
a UART chip. Most internal modems contain their own serial port
and do contain a UART chip.
Any external modem that will be used at speeds of 28Kbps or above
must be connected to a 16550A-type UART or better to run at top
speeds. For best results with external ISDN terminal adapters, use
serial ports equipped with 16750 or 16950 UARTs because they support
maximum speeds in excess of 460Kbps.
Modem Modulation Standards
Modems are frequently identified by their protocols. Use Table 6.8
to determine the speed and other characteristics of a particular protocol.
Most modems support multiple protocols.
Table 6.8 Modem Modulation Standards and Transmission Rates
Maximum Transmission
Protocol Rate (bps) Duplex Mode
Bell 103 300bps Full
CCITT V.21 300bps Full
Bell 212A 1200bps Full
ITU V.22 1200bps Half
ITU V.22bis 2400bps Full
ITU V.23 1,200/75bps Pseudo-Full
ITU V.29 9,600bps Half
ITU V.32 9,600bps Full
9-pin Serial
loopback connectors