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SUMMARY OF THE SCOPE OF THE RS-232
STANDARD
The RS-232 standard doesn't
define everything nor does it address every conceivable situation. There
are three basic categories of issues related to RS-232:
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Some engineering
specifications are absolutely explicit.
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Voltage
levels: Voltage is referenced to a zero-volt ground and ranges
from +15 volts to -15 volts.
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Baud rate
values: A baud is a signal transition from negative to
positive, or positive to negative. In an RS-232 interface there is one
binary character encoded during each baud. The term "baud" is often
used to mean "bits per second". For the RS-232 standard, it's true
that 1bps=1baud, but that's not true for other interconnection
technologies. Notably, when a modem transmits onto the phone line at,
say, a data rate of 1200 bits per second it is encoding two bits
during each signal transition; two bits per baud.
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Shape of the
signals that represent '1's and '0's: If an engineer were using an
oscilloscope to observe the signals on an RS-232 line, they would have
to comply with a specific set of constraints.
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The
specifications for the interface circuitry: The chipset that transmits the signal is called an
RS-232 line driver. The line driver must send a signal that
meets the requirements of the standard and that is exactly what the
receiver circuitry is expecting. The circuit design parameters for the
transmit and receive components are completely specified.
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The purpose for
each wire used in the cable: There are 25
wires described in the RS-232 standard. The purpose of each wire
is explicit. Moreover, not all 25 wires are required to make a simple
connection between two devices. Your personal data assistant (PDA) or
even your wristwatch may have a tiny two-wire RS-232 interface that
allows you to backup and restore data. Based on the requirements of
the interface design, as few as two of the 25 lines may be
implemented.
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There are some
general guidelines allowing certain issues to be flexible.
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The method to
start and stop the flow of data: Two fundamental control signals
are provided that indicate the operational state of the RS-232 device
and the willingness of the device to receive characters. We might find
one implementation that requires both signals be asserted before data
is sent. In another case, a designer may decide that if a device says
it's ready to receive characters then it must be in an operational
state. In this case only one of the control signals will be monitored.
There are other control signals that indicate that the phone is off
the hook and a dial tone is being received; that the phone is ringing;
and more. The method used by a particular implementation to start and
stop the flow of data is constrained by the available choices but not
specified in absolute terms.
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The method to
coordinate the sender and receiver: Coordination includes the
recognition that a new character has arrived and that the character
transmission has stopped. Also, the baud rate used between the
transmitter and receiver must be coordinated. As with the flow of
data, there are a variety of options available but the implementation
selects the specific method.
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Some considerations
have no specifications in the standard.
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The way that
characters are represented by bits: RS-232 defines the
specifications for encoding, transmitting, receiving, and decoding
"characters" of 5, 6, 7, or 8 bits in length. If these characters are
ASCII, or EBCDIC, or graphics, or a bit-map representing a fax
document, the RS-232 standard doesn't know, or care about it.
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The type of
connector used to attach the cable ends: Originally a 25 pin
connector (called a DB-25) was the most popular. This connector is
still in wide use today; mostly on the backs of external modems,
printers, or other peripheral devices. A more recent development
(well, since the mid-1980's) has been the introduction of the 9 pin
(DB-9) serial connector now found on almost all desktop and notebook
computers. Even more recently is the proliferation of PDA's (personal
data assistants) with their very simple (often 2-wire) interfaces
using tiny phono plugs or other proprietary connectors. The RS-232
standard doesn't require any particular connector.
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