DTE-to-DCE Signaling

DTE-to-DCE Signaling

Wire Function
TXD Transmits data from the DTE to the DCE. All serial connections send their data one
bit at a time over a single transmission path. This differs from parallel transmissions
that have multiple paths. Printers, for example, send a full octet per signaling window.
TX is on pin 2.
RXD Receives data from the DCE to the DTE and is carried on pin 3. If there is a need to
cross two serial ports together, as is the case in DTE-to-DTE connections, pin 2 is
linked to 3 and vice versa.
GND The electrical ground provides a baseline for voltage changes on the TX and RX
wires. It is on pin 7.
RTS Request to send. This signal is used when the DTE would like to send data.
CTS The clear to send signal is used to inform the DTE that the DCE is ready to send data
received from the DTE.
DTR The data terminal ready wire is a modem control signaling wire, which signifies that
the DTE can accept a call from the DCE.
CD Carrier detection indicates that the local DCE has a connection to the remote DCE. It
is also a modem control wire.
RI The ring indication/indicator is used to signal the DTE device that an incoming call is
ringing the phone. On nine pin interfaces, RI is on pin 9.

The information in Table 23.1 is important to understand from a troubleshooting perspective;
however, it is also nice to know for an overview of wiring. In some instances, such as the
extension of a serial connection, it might be necessary to serially link two devices by using Category
5 cable, for example. Hoods are available to make this link, and in fact, many Cisco connections
use so few wires in serial connections that console ports are terminated with RJ-45
connections. (
Hood
is a slang term describing the plastic converter that covers the wiring as it
changes from RJ to DB connections. Another term for this is
media converter
.)

NOTE:Refer to the documentation that came with your router or switch regarding
console connections. Cisco has been inconsistent with this implementation,
sometimes requiring the use of rolled connection cables and at other times
needing straight-through patch cords. A rolled connection places pin 1 on one
end into the pin 8 position on the other end; thus, pin 2 falls into the pin 7 position,
and so forth. Straight-through connections map 1 to 1 and 2 to 2, through
to pin 8 connecting to pin 8.