Specifications

Configure Low-Speed Serial Interfaces
Configuring Serial Interfaces IC-111
Configure Low-Speed Serial Interfaces
This section describes how to configure low-speed serial interfaces. In addition to the background
information described in the “Understand Half-Duplex DTE and DCE State Machines” section,
these configuration guidelines are provided for configuring low-speed serial interfaces:
• Change between Controlled-Carrier and Constant-Carrier Modes
• Tune Half-Duplex Timers
• Change between Synchronous and Asynchronous Modes
For configuration Examples, see the “APS Configuration Examples” section at the end of this
chapter.
Understand Half-Duplex DTE and DCE State Machines
The following section describes the communication between half-duplex DTE transmit and receive
state machines and half-duplex DCE transmit and receive state machines.
Half-Duplex DTE State Machines
As shown in Figure 16, 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 ms. Transmission delays are used for debugging
half-duplex links and assisting lower-speed receivers that cannot process back-to-back frames.
Figure 16 Half-Duplex DTE Transmit State Machine
Transmit
delay
state
Transmit
state
Wait
CTS
state
Wait
transmit
finish
state
Wait
RTS drop
delay
state
Wait
CTS
drop
state
Ready
state
Set timer = transmit-delay
Timer expires
Transmission finished
Set timer = rts drop-delay
Timer expires
De-assert RTS
Set timer = cts drop-timeout
CTS de-asserted
or timer expires
To ready
state
If timer expires
increment error
counter enabled
Timer expires CTS is asserted
Continue
transmission
Start transmission
More frames
to transmit
De-assert RTS
Increment error counter
Transmit queue
no longer empty
Assert RTS
Set timer = rts timeout
No more frames to transmit
S4538