DM9101
10/100Mbps Ethernet Physical Layer Single Chip Transceiver
100Base-TX Operation
By scrambling the data, thetotal energy presented to the
The block diagram in figure 3 provides an overview of the
functional blocks contained in the transmit section.
The transmitter section contains the following functional
blocks:
cable is randomly distributed over a wide frequency range.
Without the scrambler, energy levels on the cable could
peak beyond FCC limitations at frequencies related to
repeated 5B sequences like continuous transmissionof
IDLE symbols. The scrambler output is combined with the
NRZ 5B data from the code-group encoder via an XOR
logic function. The result is a scrambled data stream with
sufficient randomization to decrease radiated emissions at
critical frequencies.
- 4B5B Encoder
- Scrambler
- Parallel to Serial Converter
- NRZ to NRZI Converter
- NRZI to MLT-3
- MLT-3 Driver
Parallel to Serial Converter
4B5B Encoder
The Parallel to Serial Converter receives parallel 5B
scrambled data from the scrambler and serializes it
(converts it from a parallel to a serial data stream).
The serialized data stream is then presented to the
NRZ to NRZI Encoder block
The 4B5B encoder converts 4-bit (4B) nibble data
generated by the MAC Reconciliation Layer into a 5-bit
(5B) code group for transmission, reference Table 1. This
conversion is required for control and packet data to be
combined in code groups. The 4B5B encoder substitutes
the first 8 bits of the MAC preamble with a J/K code-group
pair (11000 10001) upon transmit. The 4B5B encoder
continues to replace subsequent 4B preamble and data
nibbles with corresponding 5B code-groups. At the end of
the transmit packet, upon the deassertion of the Transmit
Enable signal from the MAC Reconciliation layer, the
4B5B encoder injects the T/R code-group pair (01101
00111) indicating end of frame. After the T/R code-group
pair, the 4B5B encoder continuously injects IDLEs into the
transmit data stream until Transmit Enable is asserted and
the next transmit packet is detected.
NRZ to NRZI Encoder
After the transmit data stream has been scrambled and
serialized, the data must be NRZI encoded for
compatibility with the TP-PMD standard for 100Base-TX
transmission over Category-5 unshielded twisted pair
cable.
MLT-3 Converter
The MLT-3 conversion is accomplished by converting
the data stream output from the NRZI encoder into two
binary data streams with alternately phased logic one
events.
The DM9101 includes a Bypass 4B5B conversion option
within the 100Base-TX Transmitter for support of
applications like 100 Mbps repeaters which do not require
4B5B conversion.
MLT-3 Driver
The two binary data streams created at the MLT-3
converter are fed to the twisted pair output driver which
converts these streams to current sources and alternately
drives either side of the transmit transformer primary
winding resulting in a minimal current MLT-3 signal. Refer
to figure 4 for the block diagram of the MLT-3 converter.
Scrambler
The scrambler is required to control the radiated emissions
(EMI) by spreading the transmit energy across the
frequency spectrum at the media connector and onthe
twisted pair cable in 100Base-TX operation.
Final
15
Version: DM9101-DS-F03
July 22, 1999
ETC [ ETC ]
