Following up on last night's conversation...
When there is talk about CubeSats using the PC-104 standard, what that really means is the use of the PC-104 form factor in terms of board size, mounting hole placement, and the 2x2x26 connector. CubeSats don't adhere to any of the signalling or protocol aspects of PC-104 standards (including additional connectors defined in the PC-104 standard).
There hasn't been any standards body define this form factor for Cubesats (as far as I know) so I imagine it just sort of grew organically from the first university satellites looking around for something to copy and noticed the small form-factor computer.
As correctly noted, CubeSats don't standardize the use of the pins in the PC-104 connector but it seems like component manufacturers looked around to see how others had been using pins and followed suit. Thus if PACSAT is using I2C, power, and ground pins there will be little conflict. What I've seen is CubeSat component manufacturers offer an option sheet that allows the customer to select from a set of options. Along the lines of "put I2C pins here or there or do not connect" kind of options.
I hear the comments about how "This a large connector!" and "How many pins are really needed?" and "How are they used by different components?" All that is true. At same time it's also true that if somebody is adding a component to their CubeSat it will most likely expecting boards with the PC-104 form factor. AMSAT is an exception but even then we had to partially adapt our boards to the PC-104 form factor for use by non-AMSAT organizations.
When I'm at CubeSat Developers Workshop later this month, I'll do a survey. While I will see at least one university eschew the PC-104 form factor (because they're developing ALL the components in-house and prefer a plug-in-slot configuration because it simplifies assembly a lot) I expect most will have purchased the bus (including structure, C&DH, EPS, and any ADCS) which will be in the PC-104 form factor. But I could be surprised and the tide may be turning.
Comments?
Jonathan