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Clocking
This field sets the type of clocking used to clock transmit and receive data on the serial port.
If the clock goes out-of-phase, you will receive errors.
Note
The clocking value is only relevant if the port is a
physical
DCE port (i.e., DCE cable plugged into the
submodule port). If the submodule port is a physical
DTE port, clocking will default to External.
External Clocking
If you set this value to External, clocking will be controlled by the external DCE (a DSU or
other DCE device on the other end of the cable from the submodule port). External clock-
ing is the default option when the submodule is a physical DTE device (i.e., controlled by
an external DCE device).
When the submodule is acting as a physical DTE and
• the speed is greater than 256 kbps, or
• excessive FCS errors or Aborts are being detected coming from the submodule at the
remote port or line monitor
then it is recommended that the external DCE (usually a CSU/DSU) be set to take a trans-
mit data clock from the external DTE transmit clock (TXCE).
You can set up the external DCE this way by configuring its DTE, or dataport, configura-
tion options. Set the “Transmit Clock Source” to “External.” In this mode of operation, the
transmit clock is output by the DCE device and the submodule turns it around on the
external transmit clock back to the DCE, eliminating any phase misalignment between
transmit clock and transmit data.
If the external DCE does not provide a DTE configuration option for the transmit clock
source, then try setting the “Transmit Clock Polarity” to “Invert.” Note that Invert is the
clock polarity for Transmit (not Receive) and should only be used when excessive FCS
errors or Aborts are detected.
Internal Clocking
If you set this value to Internal, clocking is controlled by the internal
DCE (the submod-
ule). Internal clocking should only be selected if the submodule is a physical DCE device
and you are using an RS-232 cable. Internal clocking is the default setting when the
submodule is a physical DCE device and an RS-232 DCE cable is connected to this port.