Cisco Systems VIP-4R/4T manual Inverting the Clock Signal, Configuring NRZI Format

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-clockcommand 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-clockcommand 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-Zwhen 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-encodingcommand. For complete command descriptions and instructions, refer to the related software documentation.

66VIP-4R/4T Installation and Configuration