Serial Clocking Modes

By default, a serial interface uses loop clocking to determine its timing source. For EIA-530 and V.35 interfaces, you can set each port independently to use one of the following clocking modes. X.21 interfaces can use only loop clocking mode.

• Loop clocking mode—Uses the DCE's receive (RX) clock to clock data from the DCE to the DTE.
• DCE clocking mode—Uses the transmit (TXC) clock, generated by the DCE specifically to be used by the DTE as the DTE's transmit clock.
• Internal clocking mode—Uses an internally generated clock. The speed of this clock is configured locally. Internal clocking mode is also known as line timing.

Both loop clocking mode and DCE clocking mode use external clocks generated by the DCE.

Figure 5 shows the clock sources for loop, DCE, and internal clocking modes.

Figure 5: Serial Interface Clocking Modes

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