Modem Signaling Data

© 2004 Cisco Systems, Inc. All rights re

Although a DB-25 serial connector has 25 pins, only 8 pins are actually used for connecting an access server (DTE) to a modem (DCE). The other 17 signals are not interesting, and are ignored. You can group the eight interesting signals into three categories according to their functionality:

Data transfer

■ Hardware flow control

■ Modem control

The figure shows the data transfer group:

■ TxD: Transmit data. The DTE transmits data to the DCE.

■ RxD: Receive data. The DTE receives data from the DCE.

■ GRD: Ground (pin 7). This pin provides the ground reference for voltage measurements. Note The signals and pins shown are for the EIA/TIA -232 specifications.

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