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Tech Tip In Figure 4-3, notice that the DSR signal is the DCD signal for the modem. In the scheme of Cisco IOS, the DCD pin on the DCE is strapped to the DSR pin on the Cisco IOS DTE side. What the Cisco IOS calls DSR is not DSR; it is DCD. The DCE's actual DSR pin and ring ignore (RI) pin are ignored by the Cisco IOS.

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):

• Hardware flow control signals (RTS, CTS)

Table 4-1 RS-232 Signal State Behavior

Signal

Signal Direction

Purpose

(Output)

DTE transmits data to DCE.

(Input)

DCE transmits received data to DTE.

(Output)

DTE uses the RTS output signal to indicate if it can receive characters into the Rx input buffer1.

The DCE should not send data to the DTE when DTR input is low (no RTS).

(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.

(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.

(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).

1. The name RTS is illogical with the function (able to receive) due to historical reasons.

1. The name RTS is illogical with the function (able to receive) due to historical reasons.

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