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Datasheet
39
Networkin
g
Silicon — GD82559ER
6.1.2.3 100BASE-TX Transmit Framin
g
The PHY unit does not differentiate between the fields of the MAC frame containing preamble,
Start of Frame Delimiter, data and Cyclic Redundancy Check (CRC). The PHY unit encodes the
first byte of the preamble as the “JK” symbol, encodes all other pieces of data according to the 4B/
5B lookup table, and adds the “TR” code after the end of the packet. The PHY unit scrambles and
serializes the data into a 125 Mbps stream, encodes it as MLT-3, and drives it onto the wire.
6.1.2.4 Transmit Driver
The transmit differential pair lines are implemented with a digital slope controlled current driver
that meets the TP-PMD specifications. Current is sinked from the isolation transformer by the TDP
and TDN pins. The conceptual transmit differential waveform for 100 Mbps is illustrated in the
following figure.
The magnetics module that is external to the PHY unit converts I
TDP
and I
TDN
to the 2.0 V
pp
, as
required by the TP-PMD specification. The same magnetics used for 100BASE-TX mode should
also work in 10BASE-T mode. The following is a list of current magnetics modules available from
several vendors:
Fi
g
ure 13. NRZ to MLT-3 Encodin
g
Dia
g
ram
Clock
NRZ
NRZ1
MLT-3
1100 0110
1100 0110
1100 0110
Fi
g
ure 14. Conce
p
tual Transmit Differential Waveform
(V
TDP
-V
TDN
)
+1V
0V
-1V
t
Table 4. Ma
g
netics Modules
Vendor Model/Type 100BASE-TX 10BASE-T
Delta LF8200A Yes Yes
Pulse En
g
ineerin
g
PE-68515 Yes Yes
Pulse En
g
ineerin
g
H1012 Yes Yes