User guide

A-38 Cisco 2500 Series Access Server User Guide
Modems
RJ-45 Cabling
The access server uses high-density, 68-pin connectors with an 8-connector breakout cable
(with RJ-45 connectors) for its asynchronous ports. To connect to a modem, an MMOD
type RJ-45-to-DB-25 adapter is used. The MMOD adapter is labelled “MODEM.” (For
connecting a modem on the auxiliary port, use an RJ-45 roll-over cable and an MMOD
adapter.) For additional information about supported cabling configurations, see the
appendix “Cable Specifications.”
Note RJ-45 cabling for EIA/TIA-232 is not standardized and may vary if purchased from
different vendors. The RJ-45-to-DB25 adapters modify the Cisco RJ-45 standard to the
DB-25 standard.
EIA/TIA-232 Pairs
The EIA/TIA-232 cabling uses three pairs of wires (plus grounding) to connect the modem
(DCE) to the access server (DTE). In each pair, one wire is used as output and one as input
on each end. The three wire pairs are used for the following purposes:
• Data transfer (TX/RX)
• Hardware flow control (RTS/CTS)
• Modem control (DTR/DCD)
Table A-6 lists the signal flow between the two devices and their function for modem
communications.
Table A-6 EIA/TIA-232 Signals and Functions
Signal Name
Asynchronous
Port (DTE)
Modem
(DCE) Function
Transmit Data (TX) Output Input DTE transmits data to DCE.
Receive Data (RX) Input Output DCE transmits received data to DTE.
Request To Send
(RTS)
Output Input DTE signals to DCE it can continue to
accept data into its buffers.