Specifications

4-5
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Background on Asynchronous Data Communications
Table 4-1 describes how Cisco uses it’s RS-232 pins. The signal direction in the table is from the
perspective of the DTE (IOS line):

Data signals (TxD, RxD)

Hardware flow control signals (RTS, CTS)

Modem signals (DTR, DSR, DCD, RI)
Table 4-1 RS-232 Signal State Behavior
Signal Signal Direction Purpose
Transmit Data (TxD) ——>
(Output)
DTE transmits data to DCE.
Receive Data (RxD) <——
(Input)
DCE transmits received data to DTE.
Request To Send (RTS) ——>
(Output)
DTE uses the RTS output signal to indicate if it can
receive characters into the Rx input buffer
1
.
The DCE should not send data to the DTE when DTR
input is low (no RTS).
1. The name RTS is illogical with the function (able to receive) due to historical reasons.
Clear To Send (CTS) <——
(Input)
DCE signals to DTE that it can continue to accept data
into its buffers.
DCE asserts CTS only if the DCE is able to accept
data.
Data Terminal Ready
(DTR)
——>
(Output)
DTE signals to DCE that it can continue to accept data
into its buffers.
DTE asserts RTS only if the DTE is able to accept
data.
Data Carrier Detect
(DCD)
<——
(Input)
DCE indicates to DTE that a call is established with a
remote modem. Dropping DCD terminates the
session.
DCD will be up on the DCE only if the DCE has
achieved data mode with its peer DCE (client
modem).