As shown in the
figure below, the half-duplex DTE transmit state machine for low-speed
interfaces remains in the ready state when it is quiescent. When a frame is
available for transmission, the state machine enters the transmit delay state
and waits for a time period, which is defined by the
half-duplex
timer
transmit-delay command. The default is 0
milliseconds. Transmission delays are used for debugging half-duplex links and
assisting lower-speed receivers that cannot process back-to-back frames.
Figure 3. Half-Duplex DTE
Transmit State Machine
After idling for a
defined number of milliseconds (ms), the state machine asserts a request to
send (RTS) signal and changes to the wait-clear-to-send (CTS) state for the DCE
to assert CTS. A timeout timer with a value set by the
half-duplex
timer
rts-timeout command starts. The 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 before the timer
expires, 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 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-timeout interface 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 the
figure below, 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 4. 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
interfaces command for the serial interface in
question.