ISA bus throughput

It's not. IDE and ST506 are very different interfaces. IDE ias a formatted interface. You transfer nice 8-bit bytes and 16 bit words to the drive. The drive takes care of turning them into a suitable pulse train to send ot thehead, etc. ST506 is a low-level, raw, interface. What you see on the interface connectoer is essentially the pulse stream to/from the head. It's up to the cotnroller to turn that into the user data bytes/words. The origianl PC/AT controller talked to an ST506 drive. But of course it also had a host-side interface -- it plugged into an ISA slot and appeared as a number of I/O ports to the 286 (or whatever) processor. The controller card did the translation betweenn the user bytes/words and the pulse stream on the ST506 interface connector. Now you could also split the system up in a different way. Specifically, move most of the controller over to the drive. The 'controller card' now becomes little more than an address decoder which lets the host processor talk to I/O port registers physically located in the drive. To the host, it looks the same as the old ST506 controller (you send the same bytes to the same ports to read a particular sector). Inside the drive, who knows what goes on. That's IDE. You will have a lot of fun (for suitable values of fun) trying to get most old hard disk controllers or their hosts talking to IDE drives. Just because an IDE drive looks the same to the host as one particular ST506 controller doesn't mean that all ST506 controllers look the same. They don't. -tony