FW16 - why two expansion bay interposers?

[strobert]

so, I didn’t realize there were two interposer’s until I was reading about the (in dev progress) OcuLink Expansion Bay:

OcuLink Expansion Bay Module

We’re starting to see OcuLink-based eGPUs come around. The Expansion Bay module is sized in a way that it’s possible to fit an OcuLink receptacle in it, so a mostly passive adapter board is probably possible. This is assuming sufficient signal integrity through the full chain of connectors and cables, but it would also be possible to stick a redriver on the card. This probably isn’t something we would build ourselves, but all the necessary parts should be available in small quantities, and it…

and it mentions if you got the shell you have to get the graphics card interposer:

Framework

And for ref, the shell interposer:

Framework

I went back and re-read the interposer article/deep dive:

Framework Laptop 16 Deep Dive - Connectors

[Holding a Framework Laptop 16 touchpad] This is likely our last Framework Laptop 16 Deep Dive before we start shipping, and those of you who ordered one can dive deep on your own. We began mass production of Mainboards last week, which we’ll hold onto as we resolve the last few remaining open items to begin full system manufacturing. You may be thinking, do we really need a deep dive on connectors? The answer is a resounding “Yes!”, as connectors are surprisingly among the most complex and c…

and I didn’t see a mention of having two (one for the shell, one for graphics module). I get the shell probably doesn’t need all of the connections, but wondering why you can’tshouldn’t just use the graphics interposer even with the shell connected? or to put another way, why did framework make two interposer models? I’m figuring there is a good reason, so really just looking for what that reason is.

I looked around forums and didn’t see this discussed/asked at all (if I missed the topic, please point me at it/merge the topic).

I did see that if I do in the future decide to get the dGPU expansion bay (I bought the shell expansion bay with my FW16) that it does come with the graphics interposer:

Does the GPU module come with an interposer?

I got my FW16 without the GPU. If I were to purchase the Graphics Module from the marketplace, would it come with the interposer needed to connect it to the mainboard, or would I need to purchase that separately? I can’t find any information that says it comes with it, and the listing doesn’t say anything either. I’d hate to buy the module (when it comes back in stock) just to have to wait another week to get an interposer, but would hate spending an extra $30 if it does come with one.

[MJ1]

The GPU interposer requires a lot of tiny and fairly delicate pins. Framework mentioned that they had to improve the design to get an acceptable life, since the original simpler version sometime didn’t even last two cycles.

I guess they see a benefit in having a simpler interposer when all the pins aren’t needed. Less cost, so they can price the non-GPU bay lower. Less tiny pins, so it can tolerate not-so-gentle handling by some users better, leading to reduced customer service requests.

[lbohn]

There are some pins carrying +3V3 on the signal side of the graphics interposer. If the graphics interposer were used with the expansion shell, having those current carrying pins exposed to short against the shell case, random debris, whatever and electrically backfeed into the mainboard PCIe lanes would probably not be desirable.

[strobert]

I was wondering it may have been something like this. AKA, with the basic shell you might have exposed pins that could lead to “bad things”.

So thinking then with the OcuLink module those pins are now getting connections and thus the need for the graphics module style interposer.

[lbohn]

The left side of the interposer carries power, no data. The right side carries the data signals. The expansion bay shell only needs power to drive the fans. High-speed data signals are used for PCIe data endpoints such as the dGPU, the forthcoming M.2 carrier or the much fabled OcuLink expansion.