
KS8999
Micrel
KS8999
50
January 2005
MLT3 Coding
For 100BaseTX operation the NRZI (Non-Return to Zero Invert on ones) signal is line coded as MLT3. The net result of using
MLT3 is to reduce the EMI (Electro Magnetic Interference) of the signal over twisted pair media. In NRZI coding, the level
changes from high to low or low to high for every “1” bit. For a “0” bit there is no transition. MLT3 line coding transitions through
three distinct levels. For every transition of the NRZI signal the MLT3 signal either increments or decrements depending on
the current state of the signal. For instance if the MLT3 level is at its lowest point the next two NRZI transitions will change the
MLT3 signal initially to the middle level followed by the highest level (second NRZI transition). On the next NRZI change, the
MLT3 level will decrease to the middle level. On the following transition of the NRZI signal the MLT3 level will move to the lowest
level where the cycle repeats. The diagram below describes the level changes. Note that in the actual 100BaseTX circuit there
is a scrambling circuit and that scrambling is not shown in this diagram.
A
3
8
E
9
4
T3
R3
I1
1010 0011 1000 1110 1001 0100 UUUU UUUU UUUU UUUU
10110101011001011100100110101001101001111111111111
Hex Value
Binary 4B
Binary 5B
NRZ
NRZI
MLT3
Figure 20. MLT3 coding
Field
Octect Length
Description
Preamble/SFD
8
Preamble and Start of Frame Delimiter
DA
6
48-bit Destination MAC Address
SA
6
48-bit Source MAC Address
Length
2
Frame Length
Protocol/Data
46 to 1500
Higher Layer Protocol and Frame Data
Frame CRC
4
32-bit Cyclical Redundancy Check
ESD
1
End of Stream Delimiter
Idle
Variable
Inter Frame Idles
Table 14. MAC Frame
MAC Frame
The MAC (Media Access Control) fields are described in the table below.