Preliminary Data Sheet

AL2100

11/11/02

Broa d c om Co rp o r a ti o n

Document

AL2100-DS01-405-R

100Base-Twisted Pair PHY

A 100Base-TX data stream is not always DC balanced because the receive signal must pass through a transformer. The

DC offset of the differential receive input can wander. This effect, known as baseline wander, can greatly reduce the noise

immunity of the receiver. The 100Base-TX PHY automatically compensates for baseline wander by removing the DC offset

from the input signal, and thereby significantly reduces the chance of a receive symbol error.

The multimode transmitter transmits MLT3 coded symbols in100Base-TX mode, NRZI coded symbols in 100Base-FX mode.

It uses a current drive output, which is well balanced, and produces very low noise transmit signals. PECL voltage levels are

produced with resistive terminations in 100base-FX mode.

In 100Base-TX mode, the transmit data stream is scrambled to reduce radiated emissions on the twisted pair cable. The

data is scrambled by exclusive ORing the NRZ signal with the output of an 11-bit wide linear feed back shift register (LFSR),

which produces a 2047-bit non repeating sequence. The scrambler reduces peak emission by randomly spreading the signal

energy over the transmit frequency range, and eliminating peaks at certain frequencies.

The receiver descrambles the incoming data stream by exclusive ORing it with the same sequence generated at the

transmitter. The descrambler detects the state of transmit LFSR by looking for a sequence representing consecutive idle

codes. The descrambler locks to the scrambler state after detecting a sufficient number of consecutive idle code groups.

The receiver does not attempt to decode the data stream unless the descrambler is locked. When locked, the descrambler

continuously monitors the data stream to make sure that it has not lost synchronization.

The receive data stream is expected to contain interpacket idle periods. If the descrambler does not detect enough idle code

within 724

µs, it becomes unlocked, and the receive decoder is disabled. The descrambler is always forced into the unlock

state when a link failure condition is detected.

This feature detects the required cable connection type straight through or crossed over, and makes corrections

automatically.

Note

The baseline wander circuit is not required in 100Base-FX PHY.

Note

The stream cipher descrambler is not used in the 100Base-FX mode.