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66 VIP-4R/4T Installation and Configuration
VIP Port Adapter Functions
Following are the acceptable clockrate settings:
1200, 2400, 4800, 9600, 19200
38400, 56000, 64000, 72000, 125000
148000, 500000, 800000, 1000000, 1300000, 2000000
Speeds above 64 kbps (64000) are not appropriate for EIA/TIA-232. On all interface types, faster
speeds might not work if your cable is too long.
Inverting the Clock Signal
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 an 4T port adapter DCE port is reporting a high
number of error packets, a phase shift might be the problem. Inverting the clock can often correct
this shift.
When the 4T port adapter interface is a DTE, the invert-transmit-clock command inverts the TxC
signal it receives from the remote DCE. When the 4T port adapter interface is a DCE, this command
inverts the clock signal to the remote DTE port. Use the no invert-transmit-clock command to
change the clock signal back to its original phase.
Configuring NRZI Format
All interfaces support both NRZ and NRZI formats. Both formats use two different voltage levels
for transmission. NRZ signals maintain constant voltage levels with no signal transitions (no return
to a zero voltage level) during a bit interval and are decoded using absolute values (0 and 1). NRZI
uses the same constant signal levels but interprets the presence of data at the beginning of a bit
interval as a signal transition and the absence of data as no transition. NRZI uses differential
encoding to decode signals rather than determining absolute values.
NRZ format, the factory default on all interfaces, is most common. NRZI format, which is
configured with a software command, is commonly used with EIA/TIA-232 connections in IBM
environments.
To enable NRZI encoding on any interface, specify the slot and port address of the interface followed
by the command nrzi-encoding. Enter Ctrl-Z when you have finished with the configuration
change. In the example that follows, the first serial port on a 4T port adapter in interface processor
slot 3 is configured for NRZI encoding:
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface serial 3/1/0
Router(config-int)# nrzi-encoding
Router(config-int)# ^Z
To disable NRZI encoding on a port, specify the slot and port address and use the no nrzi-encoding
command. For complete command descriptions and instructions, refer to the related software
documentation.