Altera 8B10B Encoder/Decoder MegaCore Function - Manual (Page 20)

3–2 Chapter 3: Specifications
Functional Description
8B10B Encoder/Decoder MegaCore Function User Guide May 2011 Altera Corporation
The running disparity error output ( rderr ) is asserted when any of the following rules
apply:
■ The current running disparity is positive and the 6-bit group has more ones than
zeros or is 111000.
■ The current running disparity is negative and the 6-bit group has more zeros than
ones or is 000111.
■ The running disparity after 6-bit group is positive and the 4-bit group has more
ones than zeros or is 1100.
■ The running disparity after 6-bit group is negative and the 4-bit group has more
zeros than ones or is 0011.
1
rderr is asserted for some invalid 10-bit codes and not for others, strictly
based on the rules stated above. The computation of rderr is completely
independent of that of the special control character error ( kerr ) signal.
A 10-bit code that corresponds to a valid encoding but that has the wrong
disparity—though technically an invalid code—does not cause the
kerr
signal to be asserted. Only rderr is asserted.
f For details on running disparity rules, refer to the IEEE 802.3z specification, paragraph
36.2.4.4.
Generic Framing Procedure
The 8B10B Encoder/Decoder MegaCore function can be used within generic framing
procedure (GFP) applications. See
Figure 3–2 on page 3–2 for an example.
On ingress to the transport network, if the decoder receives an unrecognized
codeword, such as an illegal codeword or a legal codeword with a running disparity
error, it asserts the
kerr or rderr signals respectively. By asserting these error signals,
the decoder indicates to the mapper that an invalid codeword has been received, the
mapper then generates a special control character, the 10B_ERR code. In addition, the
mapper remaps the 8B/10B codewords into 64B/65B codewords before sending the
data to the transport network.
On egress from the transport network, the demapper decodes the 64B/65B codewords
and sends them to the 8B/10B encoder. When the encoder receives the 10B_ERR code,
it sends out one of the two 10-bit illegal codewords with neutral disparity: 001111
0001(RD-) or 110000 1110(RD+), depending on the running disparity.
Figure 3–2. 8B10B Encoder/Decoder GFP Typical Application
(64B/65B Encoded)
Transport Network
GFP Data Stream
Demapper
GFP
Encoder
8B/10B
Decoder
8B/10B
Mapper
GFP
Gigabit
Ethernet
Stream
(8B/10B Encoded)
Gigabit
Ethernet
Stream
(8B/10B Encoded)
Ingress Egress