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Datasheet 15
Networking Silicon — 82555
maintained (either positive, negative or zero). When an NRZ “1” arrives at the input of the encoder,
the output steps to the next level. The order of steps is negative-zero-positive-zero which continues
periodically. The figure below illustrates this process.
4.2.3 100BASE-TX Transmit Framing
The 82555 does not differentiate between the fields of the MAC frame containing preamble, start
of frame delimiter, data and Cyclic Redundancy Check (CRC). When TXEN is asserted, the 82555
accepts data on the MII TXD[3:0] lines, encodes it, and sends it out onto the wire. The 82555
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 (de-
assertion of TXEN). The 82555 scrambles and serializes the data into a 125 Mbps stream, encodes
it as MLT-3, and drives it onto the wire. If TXERR is asserted during transmission of valid data, the
82555 transmits an invalid “H” symbol.
Figure 6. NRZ to MLT-3 Encoding Diagram
Clock
NRZ
NRZ1
MLT-3
1100 0110
1100 0110
1100 0110