Electronic – Why SATA is simplex even though it has separate TX and RX pairs

Wow, I would've expected the 133MHz ATA/IDE interface would be too fast for an ATmega chip, but it looks like they use it in ATA33 (33MHz) mode, somehow getting it to work with a 4MHz(!) ATmega32.

In this case you shouldn't have any problems with a SATA to IDE converter, since they're designed to support older ATA33 devices.

Note however that the OS SCSI stack is only used for SATA devices. IDE uses a different command set, so you won't see SCSI commands arriving at your ATmega, if that's your intention. (The host PC will certainly issue SCSI commands though, and the SATA to IDE device will translate.)

An alternative that might be easier to achieve is to use a USB interface chip to make your ATmega appear as a USB hard drive. AFAIK this does use the SCSI protocol (perhaps a limited subset of it though), so you would see SCSI commands arriving on your device. Recent PCs can boot off a USB hard drive too, if that's your need for connecting directly to SATA.

This solution would be much less sensitive to timing too, as the USB chip would talk over the USB bus at the correct speeds, allowing you to talk to the chip at whatever speed you can manage.

COM ports are the common name for EIA-232 ports (aka RS-232), i.e. ports where only the lower levels of communication are defined: physical (mechanical and electrical) and data link.

For SATA these are different, and SATA defines also network and transport layers, which means packaging and error detection/correction. You can run a protocol on CMO ports which do this too, but they're not part of the COM specification itself.