2.1 Modem Functions
2.1.2 Modem signaling and cabling
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.