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In 10GBASE-R specification, Gearbox is defined to take care the data width adaptation between PCS & PMA. For example, after the 64b66b encoding, the PCS data width will become 66bits.

The gearbox will adapt the 66-bit datawidth to whatever data width that PMA is support.

May i know if we can have a PMA with serializer factor of 66. Does that mean Gearbox is not necassary or not needed in this case?

Marcus Müller
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Learner
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Sure, if the width is the same, you won't need a gearbox. But most high performance serializers are going to be a power of two, and 66 is not a power of two, so you're pretty much guaranteed to need a gearbox.

alex.forencich
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  • Yes, i understand that serializer factor of 66 does'nt make sense. Just want to verify my understanding. However, FPGA vendor like altera (Intel) does support 20 & 40 serializer factor. I think mainly is due to 8b10b reason. – Learner Oct 04 '19 at 07:47
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    That's a good point, but 5 is a relatively reasonable non power of two factor to handle. Serializers supporting 8b10b usually support 5*2^n widths. 66, on the other hand, is 2*3*11, which needs two different oddball factors, which is not so nice to implement. – alex.forencich Oct 04 '19 at 07:57
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I randomly saw this. So even IEEE maybe says something about direct connection.. But I am really new to this so correct me :D

enter image description here

pgapga
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  • This does not provide an answer to the question. Once you have sufficient [reputation](https://electronics.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://electronics.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/319658) – Jens Jun 20 '22 at 13:06
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    It is the IEEE standard 802.3 for PCR -R . It shows an indication that Gearboxes are not always used. The interface from PCS to PMA should be at least able to use the XSBI format. – pgapga Jun 21 '22 at 06:27