Specifications

Configure Low-Speed Serial Interfaces
IC-114
Cisco IOS Interface Configuration Guide
4 The DCE transitions to the wait DCD drop delay state. This state causes a time delay between
the transmission of the last frame and the deassertion of DCD in the controlled-carrier mode for
DCE transmits.
5 When the timer expires, the DCE deasserts DCD and transitions back to the ready state and stays
there until there is a frame to transmit on that interface.
As shown in Figure 19, the half-duplex DCE receive state machine idles in the ready state when it is
quiescent. It transitions out of this state when the DTE asserts RTS. In response, the DCE starts a
timer with the value cts-delay. This timer delays the assertion of CTS because some DTE interfaces
expect this delay. (The default value of this timer is 0 ms; use the half-duplex timer cts-delay
interface configuration command to specify a delay value.)
Figure 19 Half-Duplex DCE Receive State Machine
When the timer expires, the DCE state machine asserts CTS and transitions to the receive state. It
stays in the receive state until there is a frame to receive. If the beginning of a giant frame is received,
it transitions to the in giant state and keeps discarding all the fragments of the giant frame and
transitions back to the receive state.
Transitions back to the ready state occur when RTS is deasserted by the DTE. The response of the
DCE to the deassertion of RTS is to deassert CTS and go back to the ready state.
Change between Controlled-Carrier and Constant-Carrier Modes
The half-duplex controlled-carrier command enables you to change between controlled-carrier
and constant-carrier modes for low-speed serial DCE interfaces in half-duplex mode. Configure a
serial interface for half-duplex mode by using the half-duplex command. Full-duplex mode is the
default for serial interfaces. This interface configuration is available on Cisco 2520 through
Cisco 2523 routers.
Controlled-carrier operation means that the DCE interface will have DCD deasserted in the
quiescent state. When the interface has something to transmit, it will assert DCD, wait a
user-configured amount of time, then start the transmission. When it has finished transmitting, it will
again wait a user-configured amount of time, then deassert DCD.
CTS
delay
state
In giant
state
Process
received
frame
Complete frame
received
Receive
state
Ready
state
Set timer = cts-delay
Timer expires
Incomplete frame received
Drop
frame
Drop frame
End of giant
frame received
Drop frame
increments giant counter
End of giant frame
not received
RTS is asserted
Assert CTS
S4541
RTS de-asserted,
de-assert CTS
RTS is de-asserted