Specifications

Understand Half-Duplex DTE and DCE State Machines
Configuring Serial Interfaces IC-113
Figure 18 Half-Duplex DCE Transmit State Machine
After the transmit delay state, the next state depends on whether the interface is in constant-carrier
mode (the default) or controlled-carrier mode.
If the interface is in constant-carrier mode, it passes through the following states:
1 The state machine passes to the transmit state when the transmit-delay timer expires. The state
machine stays in the transmit state until there are no more frames to transmit.
2 When there are no more frames to transmit, the state machine passes to the wait transmit finish
state, where it waits for the transmit FIFO to empty.
3 Once the FIFO empties, the DCE passes back to the ready state and waits for the next frame to
appear in the output queue.
If the interface is in controlled-carrier mode, the interface performs a handshake using the data
carrier detect (DCD) signal. In this mode, DCD is deasserted when the interface is idle and has
nothing to transmit. The transmit state machine transitions through the states as follows:
1 After the transmit-delay timer expires, the DCE asserts DCD and transitions to the DCD-txstart
delay state to ensure a time delay between the assertion of DCD and the start of transmission. A
timer with the value dcd-txstart-delay is started. (This timer has a default value of 100 ms; use
the half-duplex timer dcd-txstart-delay interface configuration command to specify a delay
value.)
2 When this delay timer expires, the state machine transitions to the transmit state and transmits
frames until there are no more frames to transmit.
3 After the DCE transmits the last frame, it transitions to the wait transmit finish state, where it
waits for transmit FIFO to empty and the last frame to transmit to the wire. Then DCE starts a
delay timer with the value dcd-drop-delay. (This timer has the default value of 100 ms; use the
half-duplex timer dcd-drop-delay interface configuration command to specify a delay value.)
DCD
drop delay
state
Timer expires To ready
state
De-assert DCD
No more frames to transmit
Transmit
delay
state
Transmit
state
DCD-
txstart
delay
state
Wait
transmit
finish
state
Ready
state
Set timer = transmit-delay
Transmission finished
[controlled-carrier mode]
Transmission finished
[constant-carrier mode]
Set timer = dcd-drop-delay
Timer expires
[controlled-carrier mode]
Timer expires
[constant-carrier mode]
Start transmission
Timer expires
Continue
transmission
Start transmission
More frames
to transmit
Transmit queue
no longer empty
Assert DCD
Set timer = dcd-txstart-delay
S4540