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Digital Communication Tutorials | |||||||||||||||
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| Understanding EIA RS-232? | |||||||||||||||
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In the
early
1960s, a
standards
committee,
today
known as
the
Electronic
Industries
Association,
developed
a common
interface
standard
for data
communications
equipment.
At that
time, data
communications
was
thought to
mean
digital
data
exchange
between a
centrally
located
mainframe
computer
and a
remote
computer
terminal,
or
possibly
between
two
terminals
without a
computer
involved.
These
devices
were
linked by
telephone
voice
lines, and
consequently
required a
modem at
each end
for signal
translation.
While simple in concept, the many opportunities for data error that occur when transmitting data through an analog channel require a relatively complex design. It was thought that a standard was needed first to ensure reliable communication, and second to enable the interconnection of equipment produced by different manufacturers, thereby fostering the benefits of mass production and competition. From
these
ideas, the
RS232
standard
was born.
It
specified
signal
voltages,
signal
timing,
signal
function,
a protocol
for
information
exchange,
and
mechanical
connectors. | |||||||||||||||
| Pin assignments | |||||||||||||||
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If the
full
EIA232
standard
is
implemented
as
defined,
the
equipment
at the far
end of the
connection
is named
the DTE
device
(Data
Terminal
Equipment,
usually a
computer
or
terminal),
has a male
DB25
connector,
and
utilizes
22 of the
25
available
pins for
signals or
ground.
Equipment
at the
near end
of the
connection
(the
telephone
line
interface)
is named
the DCE
device
(Data
Circuit-terminating
Equipment,
usually a
modem),
has a
female
DB25
connector,
and
utilizes
the same
22
available
pins for
signals
and
ground.
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| DTE Connections | |||||||||||||||
| Here is the full EIA232 signal definition for the DTE device (usually the PC). The most commonly used signals are shown in bold: View Picture | |||||||||||||||
| DCE Connections | |||||||||||||||
| This shows the full EIA232 signal definition for the DCE device (usually the modem). The most commonly used signals are shown in bold: View Picture | |||||||||||||||
| DB-9 Standard Cable: | |||||||||||||||
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This shows
a 9-pin
DTE-to-DCE
serial
cable that
would
result if
the EIA232
standard
were
strictly
followed.
All 9 pins
plus
shield are
directly
extended
from DB9
Female to
DB9 Male.
There are
no
crossovers
or
self-connects
present.
Use this
cable to
connect
modems,
printers,
or any
device
that uses
a DB9
connector
to a PC's
serial
port.
This cable may also serve as an extension cable to increase the distance between a computer and serial device. View Picture | |||||||||||||||
| DB9 Loopback Connector | |||||||||||||||
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A loopback
connector
usually
consists
of a
connector
without a
cable and
includes
internal
wiring to
reroute
signals
back to
the
sender.
This DB9
female
connector
would
attach to
a DTE
device
such as a
personal
computer.
When the
computer
receives
data, it
will not
know
whether
the
signals it
receives
come from
a remote
DCE device
set to
echo
characters,
or from a
loopback
connector.
Use
loopback
connectors
to confirm
proper
operation
of the
computer's
serial
port. Once
confirmed,
insert the
serial
cable you
plan to
use and
attach the
loopback
to the end
of the
serial
cable to
verify the
cable.
In this case, Transmit Data joins to Received Data, Request-to-Send joins to Clear-to-Send, and DTE-Ready joins to DCE-Ready and Received Line Signal Detect. View Picture | |||||||||||||||
| DB9 Null Modem Cable | |||||||||||||||
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Use this
female-to-female
cable in
any
application
where you
wish to
connect
two DTE
devices
(for
example,
two
computers).
A
male-to-male
equivalent
of this
cable
would be
used to
connect
two DCE
devices.
The cable shown here is intended for RS232 asynchronous communications (most PC-based systems). View Picture | |||||||||||||||
| Useful Connection Options | |||||||||||||||
| DB25 to DB9 Adapter | |||||||||||||||
| DB9 to DB25 Adapter | |||||||||||||||
| DB25 All-Line Direct Extension | |||||||||||||||
| DB25 Loopback Connector | |||||||||||||||
| DB25 Null Modem (no handshaking) | |||||||||||||||
| DB25 Null Modem (standard handshaking) | |||||||||||||||
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