Specifications

Configure Low-Speed Serial Interfaces
IC-112
Cisco IOS Interface Configuration Guide
After idling for a defined number of milliseconds, the state machine asserts a request to send (RTS)
signal and changes to the wait-clear-to-send (CTS) state for the data communications equipment
(DCE) to assert CTS. A timeout timer with a value set by the half-duplex timer rts-timeout
command starts. This default is 3 ms. If the timeout timer expires before CTS is asserted, the state
machine returns to the ready state and deasserts RTS. If CTS is asserted prior to the timer’s
expiration, the state machine enters the transmit state and sends the frames.
Once there are no more frames to transmit, the state machine transitions to the wait transmit finish
state. The machine waits for the transmit first in first out (FIFO) in the serial controller to empty,
starts a delay timer with a value defined by the half-duplex timer rts-drop-delay interface
command, and transitions to the wait RTS drop delay state.
When the timer in the wait RTS drop delay state expires, the state machine deasserts RTS and
transitions to the wait CTS drop state. A timeout timer with a value set by the half-duplex timer
cts-drop-timeoutinterface command starts, and the state machine waits for the CTS to deassert. The
default is 250 ms. Once the CTS signal is deasserted or the timeout timer expires, the state machine
transitions back to the ready state. If the timer expires before CTS is deasserted, an error counter is
incremented, which can be displayed by issuing the show controllers command for the serial
interface in question.
As shown in Figure 17, a half-duplex DTE receive state machine for low-speed interfaces idles and
receives frames in the ready state. A giant frame is any frame whose size exceeds the maximum
transmission unit (MTU). If the beginning of a giant frame is received, the state machine transitions
to the in giant state and discards frame fragments until it receives the end of the giant frame. At this
point, the state machine transitions back to the ready state and waits for the next frame to arrive.
Figure 17 Half-Duplex DTE Receive State Machine
An error counter is incremented upon receipt of the giant frames. To view the error counter, use the
show interface command for the serial interface in question.
Half-Duplex DCE State Machines
As shown in Figure 18, for a low-speed serial interface in DCE mode, the half-duplex DCE transmit
state machine idles in the ready state when it is quiescent. When a frame is available for transmission
on the serial interface, such as when the output queues are no longer empty, the state machine starts
a timer (based on the value of the transmit-delay command, in milliseconds) and transitions to the
transmit delay state. Similar to the DTE transmit state machine, the transmit delay state gives you
the option of setting a delay between the transmission of frames; for example, this feature lets you
compensate for a slow receiver that loses data when multiple frames are received in quick
succession. The default transmit-delay value is 0 ms; use the half-duplex timer transmit-delay
interface configuration command to specify a delay value not equal to 0.
In giant
state
Process
received
frame
Complete frame
received
Ready
state
Incomplete frame received
Drop
frame
Drop frame
End of giant
frame received
Drop frame
increment giants error counter
End of giant frame
not received
S4539