In another form of error checking, the transmitter sends each message twice
and the receiver verifies that the message is the same both times. Of course this
means each message takes twice as long to transmit. Sending duplicate data can
be useful when sending occasional, short bursts of data in an environment
prone to errors. Many infrared data links use this method.
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3
This chapter explores the options for COM ports on PCs, including ports in
USB/serial converters and ports in serial servers on networks.
As Chapter 1 explained, COM ports on PCs can include ports on motherboards,
expansion cards, USB converters, and serial servers. Other names for
COM ports are communications port and Comm port.
For each COM port, an operating-system driver assigns a symbolic link name
such as COM1, COM2, and so on, which applications use to detect and access
the port. Recent Windows editions don??™t limit the number of COM ports. Of
course, every system has finite resources that limit how many COM ports can
be in use at the same time.
$8
The Windows Device Manager shows information about each COM port. To
access the Device Manager, right click on My Computer, click Manage, and in
the Computer Management pane, select Device Manager. Or click Start and
Chapter 3
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select Settings > Control Panel > System > Hardware > Device Manager.
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