Draft 6/5/00
Block Diagram Description
2-15
Copyright 2000 by LSI Logic Corporation. All rights reserved.
The 4B5B encoder takes 4B (four-bit) nibbles from the Transmit MAC
block, converts them into 5B (five-bit) words according to
Table 2.4
, and
sends the 5B words to the scrambler. The 4B5B encoder also substitutes
the first eight bits of the preamble with the Start of Stream Delimiter
(SSD) (/J/K/ symbols) and adds an End of Stream Delimiter (ESD) (/T/R/
symbols) to the end of each packet, as defined in IEEE 802.3 and shown
in
Figure 2.2
. The 4B5B encoder also fills the period between packets
(idle period), with a continuous stream of idle symbols, as shown in
Figure 2.2
.
2.2.3.2 Manchester Encoder (10 Mbits/s)
The Manchester Encoder shown in
Figure 2.1
is used for 10 Mbits/s
operation. It combines clock and non-return to zero inverted (NRZI) data
such that the first half of the data bit contains the complement of the
data, and the second half of the data bit contains the true data, as
specified in IEEE 802.3. This process guarantees that a transition always
occurs in the middle of the bit cell. The Manchester encoder on the
device converts the 10 Mbits/s NRZI data from the Ethernet controller
interface into a single data stream for the TP transmitter and adds a start
of idle pulse (SOI) at the end of the packet as specified in IEEE 802.3
and shown in
Figure 2.2
. The Manchester encoding process is only done
on actual packet data; during the idle period between packets, no signal
is transmitted except for periodic link pulses.
J
SSD #1
11000
0101
K
SSD #2
10001
0101
T
ESD #1
01101
0000
R
ESD #2
00111
0000
H
Halt
00100
Undefined
---
Invalid codes
All others
1
0000
1. These 5B codes are not used. The decoder converts them
to a 4B code of 0000. The encoder converts the 4B 0000
code to the 5B 11110 code, as shown in symbol 0.
Table 2.4
4B/5B Symbol Mapping (Cont.)
Symbol Name
Description
5B Code
4B Code