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Re: Comma Characters

Dear comma fanatics,

Tom is absolutely correct. I was hoping to save everyone from this level
of detail. Here's the whole scoop on comma usage in 8B/10B transmission

comma+ = 0b0011111
comma- = 0b1100000

The comma+ and comma- nomenclature is used in the 10-bit Interface
Technical Report which is referenced by both Fibre Channel and Gigabit

The 8B/10B code supports three code-groups containing a comma. They are
/K28.1/, /K28.5/ and /K28.7/. See IEEE Std 802.3, 1998 Edition, Table

The base Gigabit Ethernet and Fibre Channel protocol supports only one
of the above code groups, /K28.5/. /K28.7/ may be employed for
diagnostic purposes as it i contains zeros and five ones. If signaled
continuously, /K28.7/ results in the lowest frequency pattern possible.

The 10-bit pattern corresponding to the /K28.5/ code-group when the
current running disparity is negative is: 0b0011111010. This code-group
is sometimes referred to as /-K28.5/. Note that /-K28.5/ contains a

The 10-bit pattern corresponding to the /K28.5/ code-group when the
current running disparity is positive is: 0b1100000101. This code-group
is sometimes referred to as /+K28.5/. Note that /+K28.5/ contains a

Best Regards,

Tom Lindsay wrote:
> 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
> suppliers.
> 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.
> Tom
> Rich Taborek wrote:
> > Brian,
> >
> > 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,
> > Rich
> >
> > --
> >
> > Brian Cruikshank wrote:
> > >
> > > In Clause 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
Richard Taborek Sr.                 Phone: 408-845-6102       
Chief Technology Officer             Cell: 408-832-3957
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