Installation guide

5-24 Cisco 4000 Series Installation Guide
Additional Configuration Tasks
Use the no clockrate command to remove the clock rate for DTE operation. Following are
the acceptable clock rate settings:
Speeds above 64 kbps (64000) are not supported for EIA/TIA-232. On all interface types,
if your cable is too long, faster speeds might not work.
Inverting the Clock Signal on Serial Interfaces
Systems that use long cables may experience high error rates when operating at higher
transmission speeds. Slight variances in cable construction, temperature, and other factors
can cause the clock and data signals to shift out of phase. If a DCE port is reporting a high
number of error packets, the problem might be caused by a phase shift. Inverting the clock
can often correct this shift.
When a port is operating in DCE mode, the default operation is for the attached DTE device
to return the serial clock transmit external (SCTE) to the DCE port. The DCE sends serial
clock transmit (SCT) and serial clock receive (SCR) clock signals to the DTE, and the DTE
returns an SCTE clock signal to the DCE. If the DTE device does not return SCTE, you
must use the dce-terminal-timing-enable command to configure the DCE port to use its
own clock signal instead of the SCTE signal that would normally be returned from the DTE
device.
To configure an interface to accept the internal clock generated by the serial module instead
of the SCTE clock that is normally returned by the DTE device, specify the interface
followed by the dce-terminal-timing-enable command. In the example that follows, the
serial 0 port is configured to accept the internal clock signal:
interface serial 0
dce-terminal-timing-enable
1200
2400
4800
9600
19200
38400
56000
64000
72000
125000
148000
500000
800000
1000000
1300000
2000000
4000000