SDIO vs ATA vs SCSI

[Swift Griggs <swiftgriggs%gmail.com@localhost>]

I've been trying to figure out the relationship of SDIO to ATA. The reason 
is to find more ATA compatible hardware for DEC Alpha machines. Lots of 
them had ATA interfaces for CDROMs or system drives. However, most appear 
to support IDE rather than EIDE. I'm basing that off the lack of a keyed 
connector with a missing pin.

Many SD2IDE devices adapt SD cards to an IDE bus. However, I've noticed 
that the systems with ACER M1543C controllers like the ones in the 
Alphastation DS10 will throw CAM errors and generally act-out, slow down, 
and misbehave (under Tru64, Linux, or BSD - I figure it's a hardware 
thing). However, systems with the CMD PCI0646 controller like the 
Alphastation 600a have no trouble whatsoever with SD2IDE devices.

So, can anyone speculate on what is going on? Here's my cockamamie theory. 
I think SDIO is only a subset of the ATA standard and that those little 
$10 gizmos are basically only changing the electrical interface. They 
aren't like a SCSI2SD that actually has to "translate" the SDIO into SCSI 
CDBs. My guess is that the ACER controller is asking the SD card ATA 
questions it doesn't know the answer to, so to speak.

Does anyone who has worked with SDIO know what the relationship to SDIO 
and ATA is? Is it the same for CF cards? I ask because CF2IDE devices work 
even worse - they show up (wrong) as WORM or CDROM drives on my Alpha's. 
I've skimmed over this document:

https://www.sdcard.org/developers/overview/sdio/sdio_spec/Simplified_SDIO_Card_Spec.pdf

However, it's difficult to get a "summary" or intuit why some SD2IDE & 
CF2IDE devices work or don't work. My fundamental question is this:

Is SDIO a superset of ATA or is ATA a superset of SDIO? Something else?

I'm also scratching my head wondering if CAM handles SDIO directly or 
deals with it via ATA (ie.. in kernel land) ?

-Swift