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In the early days of computer CD-ROM drives, aside from SCSI interfaces, there were a number of proprietary interfaces developed by individual manufacturers. This was exemplified by PC sound cards such as this Sound Blaster 16, which included three different connectors for CD-ROM drives: enter image description here (source: https://commons.wikimedia.org/wiki/File:KL_Creative_Labs_Soundblaster_16_CT2230.jpg)

On the left of the board, we see headers for a Creative / Panasonic drive, a Mitsumi drive, and also a Sony drive.

As the IDE/ATAPI standard became established, later cards were shipped with a single 'IDE' header (e.g. the Sound Blaster 32), and was eventually left off altogether when motherboards started carrying dual IDE controllers as standard.

My question is as follows:

How did these early interfaces differ from each other, and from the eventual ATAPI standard that followed?

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Physically, the Panasonic and Mitsumi interfaces are identical to unkeyed ATA interfaces, and 40-pin unkeyed IDE cables can be used to connect Panasonic and Mitsumi drives to a compatible controller. The Sony interface is based on the 34-pin floppy interface instead.

In terms of signalling, according to AllPinouts the Mitsumi and Panasonic pinouts are the same (which seems surprising to me, I’ll update if I find anything different). The Sony pinout is very similar, but squashed to 34 pins, which is quite straightforward since four pins aren’t connected in the 40-pin interfaces and half the pins are ground (all even pins on the Mitsumi, Panasonic and Sony interfaces are ground).

The ATA interface supports all ATAPI drives, not just CD-ROM drives, so it uses more pins for non-ground purposes; for example, it has 16 data lines (instead of 8 on the CD-ROM specific interfaces) and 3 address lines (instead of 2). It also includes a LED driver, a key pin etc.

All three interfaces used different controllers, and required different drivers. If you’re really interested you can look at the different implementations in older versions of the Linux kernel; they provide a unified interface which makes it easier to understand them, but it’s still a lot of work to describe the differences.

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  • What does "unkeyed" mean in this context? – hippietrail Aug 11 at 6:35
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    @hippietrail it means without a blocked pin; some ATAPI cable connectors have a blocked pin as well as the external key, so they can’t be used with connectors with all 40 pins. – Stephen Kitt Aug 11 at 7:03

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