Re: Comma Characters
Folks - one caution - not everyone recognizes the term "positive" comma. The
specific comma you refer to has more 1's than 0's (10B is 0011111010) and it
ends in positive disparity, but some folks refer to this as the negative
comma because it is taken from the negative disparity column (see FC-PH
table 23). I have seen this discrepancy in SERDES data sheets from different
So, if you decide to distinguish them, I would avoid using positive or
negative without sufficient definition. Best would be to either refer to the
running disparity column it is called from or list out the exact sequence.
Rich Taborek wrote:
> The history is that some old SerDes initially designed for Fibre Channel
> but slated for use in Gigabit Ethernet only supported one version of
> comma. I believe that you are correct in stating that the specific
> version was the positive comma version, also referred to as comma+ and
> corresponding to the bit pattern 0b0011111. The Gigabit Ethernet
> 1000BASE-X PCS protocol is designed to emit both comma versions in order
> to be "friendly" to all SerDes parts.
> Clause 48, 10GBASE-X PCS is specified to statisitically emit an equal
> number of both comma versions. The PCS implicitly requires the
> generation and detection of both comma versions. The big difference
> between 10GBASE-X and 1000BASE-X is that the 10GBASE-X does not require
> comma detection in the PMA.
> Personally, I don't believe that anything needs to be added to the
> Clause 48 to clarify this point since it is the "obvious" way that an
> 8B/10B protocol should work. Please go ahead and submit a comment if you
> feel otherwise.
> Best Regards,
> Brian Cruikshank wrote:
> > In Clause 18.104.22.168.2 on page 261, the /COMMA/ is referred to being
> > defined in clause 36.
> > In this section, both a positive and negative comma are defined.
> > I believe that in 1 GE devices, usually only positive commas were
> > recognized. Is this enough to be 1 GE and 10 GE compliant?
> > In a IPG over 20 bytes, both commas will probably exist. In
> > sustained minimum IPG, the positive comma occurrence may be random.
> > Do positive commas occur often enough?
> > Maybe this detail should be stated?
> > /Brian Cruikshank
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title:Standards & Specifications