ATA (IDE) hard disks and interfacing

   ATA hard disks are most common storage devices in modern computers. It stays mostly for PC computers, but ATA disks are easy attachable to almost any computer, so to old (retro) computers from eighties of last century too.
  Little history of ATA (IDE) disks: they appeared somewhere in late eighties, with purpose to be easy attachable to PC AT computers and be cheap. IDE is abbrev. from 'Integrated Drive Electronic'. In fact, such disks were just MFM (RLL) controllers for PC AT, integrated with hard disk itself. It was necessary because of speed and compability problems between controllers and diverse drives.
  Of course, control and register set of MFM disks is used by early IDE disks. But it caused lot of limitations too. Most known limit was max capacity - half GB, what became too little very fast.
But such drives became very popular in couple years, while interface cards for PC computers were cheap.
  However, looking some books and articles in magazines, it looks that many expert did not realised then something what we know now - that IDE will be dominant in mass storage attachment in 90-es and later.
They talked about timing limitations, 'just little cheaper than SCSI' and similar. While drives really were not much cheaper than equivalent SCSI ones, controllers for SCSI were much-much more  expensive.
It caused segregation of market - disk manufacturers started to make different drives for SCSI and IDE, not just in interface but in whole concept - SCSI drives were faster, bigger, more reliable, so intended for professional, server usage, while IDE drives were cheaper, for mass usage in home/small office area.
  IDE was originally designed as simple port for AT-Bus, what worked at 4-8MHz in those times. Instead IDE, it was called often right so: AT-Bus interface. With raising demands for more speed and capacity direct AT-Bus attachment became obsolete. New PC motherboards had integrated IDE controllers, with DMA and own timings. EIDE standard is established for new generation of IDE disks. Most important was LBA addressing, what moved capacity limit to 8GB then. And lot of new speed modes was introduced.
  For retro computer users new modes are not so relevant, because we don't need some big speeds, and buses in such machines are slow. DMA is also not interesting - it would allow speeds in ranges of 4-20MB/sec, but no real need for it. We can stay by much simpler I/O port based interfaces, which still allow good speeds - in case of Atari ST up to 1.8Mbyte/sec .

Later development of IDE: new official abbrev is now ATA (AT attachment). Support for CD ROM storage is added, in form of ATAPI (AT attachment Packet Interface).
LBA moved up to 128GB max capacity, new UDMA modes with burst speeds up to 133Mbytes/sec were introduced. Now 200-400GB capacity per drive is usual, and of course we have LBA48, with 48bit addressing, what will be enough for many years. SATA is next generation of ATA (IDE) - with more practical serial cable, but even faster transfer.
IDE exists by Flash storage cards too - Compact Flash cards have so called 'True IDE mode' - and are connectable to IDE ports of computers. Probably next one is Flash card with SATA connection....
At moment (2007) Flash cards are most interesting for retro computers  - they are now cheap, with enough capacity and speed for any old machine. Compact Flash is easy attachable, and works with old IDE interfaces too, just needs relatively cheap CF-IDE adapter.

PP ,  2007