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Chapter 4 - Advanced Data Communications
4-3
RS-232C or EIA-232D/E
RS-232C is the Recommended Standard (RS) of the Electronic Industries
Association (EIA), defining the serial communication interface between a DTE
and a DCE. The 232 is basically a serial number for the defined standard.
Sometimes it is necessary to redefine a standard, or to revise it. The most
commonly used revision of the RS232 standard is the "C" revision. For the "D"
revision, the prefix was changed to EIA. Except for a few signals which were
added but not commonly used, there is no practical difference between the "C"
and "D" revisions. There is now a new revision with the "E" suffix. The RS-232C
standard is equivalent to the ITU-T V.24 and V.28 standard.
Serial RS-232C Cable
A serial RS-232C cable is used to connect a DTE port to a DCE port. Do not use
a null-modem cable (which may be used to connect two DTEs directly with each
other through their serial ports). A normal RS-232C connector has 25 pins, and a
normal RS-232C cable has 25 wires. Many signals in the RS-232C are not used
in common applications, and a 9-wire RS232C cable is sufficient in most
applications. The PCAT's serial port has only 9 connector pins, thus eliminating
unnecessary pins. For high-speed DTE-DCE communication, use a low-
capacitance cable (as short as possible).
Communication Protocols and Speeds
The ITU-T or ITU-TSS (International Telecommunications Union -
Telecommunications Standardization Sector) is the international standard-making
body for telecommunications. Their primary function is to draft recommendations.
The recommendations they make for modem applications have a "V" prefix and
are called V-series recommendations. The most commonly used ITU-T modem
standards for 2-wire dial-up lines are summarized in the chart below.
Standard Speed (bps)
V.34 28,800 ~ 2 400
V.32bis 14,400 / 12,000 / 7,200
V.32 9,600 / 4,800
V.22bis* 2,400 / 1,200
V.22 1,200
V.21 300
V.23 1,200 / 75
* bis is the old French word for second
In the USA, Bell Systems used to create de facto standards such as Bell 212A for
1200 bps modems and Bell 103 for 300 bps modems. Everyone follows the ITU-
T standards now for newer and higher-speed modems. The Omni 288S supports
all the above mentioned modem standards and are compatible with existing
modems.