Cisco EF4116 Hardware Installation And Maintenance Manual page 44

Network Connection Considerations
Configuring the Four-Port Serial Module Interfaces
The following sections describe the commands for configuring an external clock signal for a DCE
interface and for configuring a port for NRZI encoding or 32-bit CRC. Configuration commands are
executed from the privileged level of the EXEC command interpreter.
Configuring the Four-Port Serial Module Timing (Clock) Signals
All interfaces support both DTE and DCE mode, depending on the mode of the interface cable
attached to the port. To use a port as a DTE interface, connect a DTE adapter cable to the port. When
the system detects the DTE mode cable, it automatically uses the external timing signal. To use a
port in DCE mode, you must connect a DCE interface cable and set the clock speed with the
clockrate configuration command. This section describes how to set the clock rate on a DCE port
and, if necessary, how to invert the clock to correct a phase shift between the data and clock signals.
Setting the Four-Port Serial Module Clock Rate
All DCE interfaces require a noninverted internal transmit clock signal, which is generated by the
serial module. The default operation on a DCE interface is for the DCE device to generate its own
clock signal (TXC) and send it to the remote DTE. The remote DTE device returns the clock signal
to the DCE. The clockrate command specifies the rate as a bits-per-second value. In the following
example, the clock rate for the top serial interface on a dual serial module is defined as 72 Kbps:
Use the no clockrate command to remove the clock rate for DTE operation. Following are the
acceptable clockrate settings:
1200
125000
Speeds above 64 Kbps (64000) are not supported for EIA/TIA-232. On all interface types, if your
cable lengths exceed the standard recommendations, faster speeds might not work.
Inverting the Clock Signal on the Four-Port Serial Module
Systems that use long cables may experience high error rates when operating at the 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, suspect 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 clock signal (SCTE) to the DCE port. The DCE sends SCT and SCR clock signals to the DTE,
and 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 in place 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 in place 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:
2-22 Cisco 4000 Series Hardware Installation and Maintenance
interface serial 1
clockrate 72000
2400 4800
148000 500000
interface serial 0
dce-terminal-timing-enable
9600 19200 38400
800000 1000000 1300000 2000000 4000000
56000 64000 72000