| Many different standards define the
signaling over a serial cable, including EIA/TIA-232, X.21, V.35, EIA/TIA-449,
EIA-530, and EIA-613 High-Speed Serial Interface (HSSI). Each standard
defines the signals on the cable and specifies the connector at the end
of the cable.
With the 25-pin connector of EIA/TIA-232
standard, only eight pins are actually used for connecting a DTE (such
as an access server) to a DCE (such as a modem). The other 17 signals
are not interesting and are ignored. The eight interesting signals
(pins) can be grouped into three categories by their functionality:
- Data-transfer group
- Hardware flow-control group
- Modem-control group
Figure
illustrates these three groups.
Data-Transfer Group
The data-transfer group signals and pin designation, also known as
pinout, for the EIA/TIA-232 specification in Figure
are explained in Table .
Flow-Control Group
Pins 4 and 5 form the hardware flow-control group, as shown in Figure . These signals are activated between the DCE and the DTE when the equipment is ready to accept data. Table
explains the different flow-control signals.
Modem-Control Group
Finally, the remaining interesting signals between a DTE device and a
DCE form the modem-control group, covered in Table .
These signals are used between the DTE and DCE to initiate, terminate,
and monitor the status of the connection.
Tip: In the teletype days, flow control was
done with inband signaling using Xon/Xoff. With higher DTE speeds and
faster workstations, modems and computers were not always able to
exchange this inband signaling in a timely fashion. Therefore, a set of
electrical signals was developed to manage the flow control and the
modem control.
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