Cisco Systems C819GUK9, C819HG4GVK9 manual 8-16, Half-Duplex DCE Transmit State Machine

Chapter 8 Configuring the Serial Interface

How to Configure Serial Interfaces

Figure 5 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-delaytimer 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-delaytimer 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 is started based on the value specified using the dcd-txstart-delaycommand. (This timer has a default value of 100 ms; use the half-duplex timer dcd-txstart-delayinterface 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 by specifying the value using the dcd-drop-delaycommand. (This timer has the default value of 100 ms; use the half-duplex timer dcd-drop-delayinterface configuration command to specify a delay value.)

	Cisco 819 Series Integrated Services Routers Software Configuration Guide
8-16	OL-23590-02