The EIA Standard, RS-232-C, is the interface between Data Terminal
Equipment(DTE), and Data Communication Equipment(DCE).  DTE is typically
a computer, printer, or terminal.  DCE is generally a modem.  RS232-C
is the latest version of the standard and outlines the set of rules for
exchanging data between computers and peripherals.  

It was originally designed to allow computers, terminals, or printers
to be connected to modems.  However, it has been used extensively to
connect computers to local devices, including printers, plotters, and
other computers.  Because of the change in use, cross-over cables must
be designed.  SCS grew out of this requirement.

The standard has 4 categories of leads, ground, data, control, and timing.
Protective ground is used for protection from electrical shock, with
Signal ground used as a reference for the other categories of leads.
The Data leads are used for transmitting and receiving data.  Control
leads control the interface, direction of data flow, and indicate the status
of the attached equipment.  The timing leads are used only in synchronous
environments.  Typically when printers are being attached to computers, 
these leads are not used.

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