The original IBM PC AT’s fixed disk controller was originally referred to as an AT Attachment or “ATA” controller, and it was very simple: it supported only eight commands and a maximum of two fixed disks.
As the ATA interface evolved, the original PC AT implementation became known as “ATA-1”, and in order to support more than two fixed disks, controllers typically added a second independent set of registers and control circuitry. The original registers were referred to as the “primary” interface, and the second set was called the “secondary” interface. Later, the ATA Packet Interface (“ATAPI”) was defined, along with some additional ATA controller commands, which made it possible for an ATA controller to talk to SCSI-based CD-ROM drives.
It’s unclear whether any ATAPI controllers existed when IBM was manufacturing the PC AT, but there’s no reason why an ATAPI controller for a 16-bit ISA bus would not have worked, as long as its “primary” interface was disabled (to avoid conflicting with the PC AT’s existing ATA controller) and the ATAPI drive was connected to controller’s “secondary” interface.
And in fact, that is exactly how the machine below is configured. It’s also available with the PCjs Debugger.
[PCjs Machine "ibm-5170-vga"]
Waiting for machine "ibm-5170-vga" to load....