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General: The first letter of the EIA signal name categorizes the signal into one of five groups, each representing
a different "circuit":
•
A - Ground
•
B - Data
•
C - Control
•
D - Timing
•
S - Secondary channel
•
1 Protective Ground
Name: AA
o
Direction: -
o
CCITT: 101
o
This pin is usually connected to the frame of one of the devices, either the DCE or the DTE, which is properly grounded. The
sole purpose of this connection is to protect against accidental electric shock and usually this pin should not be tied to
Ground.
This pin should connect the chassis (shields) of the two devices, but this connection is made only when
connection of chassis grounds is safe (see ground loops below) and it is considered optional.
Ground loops are low impedance closed electric loops composed from ground conductors. When two
grounded devices are connected together, say by a RS-232 cable, the alternating current on the lines in
the cable induces an electric potential across the ends of the grounding line (either Protective Ground or
Signal
Ground), and an electric current will flow across this line and through the ground.
Since the loops impedance is low, this current can be quite high and easily burn out electric components.
Electrical storms could also cause a burst of destructive current across such a loop. Therefore,
connection of the Protective Ground pin is potentially hazardous. Furthermore, not all signal grounds are
necessarily isolated from the chassis ground, and using a RS-232 interface, especially across a long
distance, is unreliable and could be hazardous. 30 meters is considered the maximum distance at which
the grounding signals can be connected safely.
•
2 Transmit Data
Name: BA
o
Direction: DTE -> DCE
o
CCITT: 103
o
Serial data (primary) is sent on this line from the DTE to the DCE. The DTE holds this line at logic 1 when no data are being
transmitted. A "On" (logic 0) condition must be present on the following signals, where implemented, before data can be
transmitted on this line : CA, CB,CC and CD
RS-232 CHARACTERISTICS
RS-232 Signals Functional Description
(Request To
Send,
32
Clear To Send,Data Set
Ready,
Signal
Data Terminal
Ready).
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