David Empson schrieb: > The original ST506 interface (as used in the ProFile, for example) uses > two ribbon cables. One is 34 pins, and is basically the same as the > floppy drive interface used in the PC. The other is about 20 pins, and > carries parallel data to/from the drive. > Unfortunately Apple didn't use a MFM drive with ST506 interface for their ProFile. They took only the Seagate ST506 drive mechanism (ST412 will work, too) and fitted it with their own analog board. This board ist the only ECL logic circuit I ever met up to now. ST506 drive and analog board are controlled by the Profile logic board in a Disk II-like way (phi0..3, RD, WD, WE, and GCR recording). Patrick "you" wrote in message news:Gaij8.260$UI4.159383@news.uswest.net... > >No one has cracked the lid and > >looked at one? > I am sitting in my shop with an open HD20 in fromt of me. Inside we find a > Power Supply a fan, a disk, and a board. The disk is a Rodime Model 552. > Connector seems to be a 26 pin affair similar to a Apple II SCSI card connector There is one other 26 pin SCSI that I know of, the one used in the Mac Portable. That's an old luggable Mac. Most of them have a wire harness soldered to a 40 meg SCSI hard disk, a few have an adapter cable that plugs into a normal SCSI connector. Here's a little info on the HD20. Couldn't find much on the net. -Paul "It cost over $1000 new in 1986 (although I believe it was introduced sometime in 1985). Inside is a 3.5" HH Rodime 552 disk, which actually has an Apple Disk Drive interface right on it. I had expected some sort of conversion circuitry (like the old Sun ESDI->SCSI boards), but there isn't. (400k) Average seek: 415 ms Rotational: 394, 429, 472, 525, 590 RPM Burst transfer: 489.6 Kbits/sec (serial) (HD20) Average seek: 85 ms Rotational: 2744 RPM Burst transfer: 500 Kbits/sec (serial) It looks like the 500kbits/sec is a limitation of the floppy interface and not of any device on it. Anyway, since the 512k only knows how to boot from the internal floppy, and speaks only MFS (the original, flat Mac File System---the folders are purely ornamental! Stupid trivia bit: folders on MFS disks have one extra pixel) on a 400k disk drive, Apple got creative with their solution: The Apple HD20 INIT (introduced with System Software 1.1) patches the ROM to allow the use of HFS (a 20 meg flat file system would be a horrible mess), the HD20, and the 800k disk drive. Insert the boot disk with 1.1 and the HD20 INIT, the Mac boots half-way, spits out the disk, and continues from the HD20. Kind of neat. On the Plus (and presumably 512ke), which speaks HFS and 800k disk drive natively, you can boot right off the HD20. But the Plus has SCSI, so this is trivial." Leon Howell wrote: > > > *W*H*Y* won't the Hard Disk 20 work with the Apple II? > > > > Because the electrical interface > > The interface is the same as the Apple ][ as far as the floppies are > concerned No it isn't. They are very closely compatibile, including having the same connector and MOST of the pins are the same, but there are some differences. For example, the early Mac floppy port has an output which is controlled by one of the multifunction outputs of its VIA chip, able to generate square waves and other signals without much software intervention. This signal is used to control the rotation speed of the 400K Mac drive (and possibly the early 800K drive) according to which track is being accessed. (This is done automatically by the Apple 3.5 Drive.) This timing signal is not available from an Apple II's disk port (except for the Universal Disk Controller) so you can't normally connect a Macintosh 400K drive to an Apple II. I'd have to go hunting to find out which pin is used for this signal, but my recollection is that is the pin used to sense the write protect status of a 5.25" drive on the Apple II. The HD20 might be dependent on using this pin for a special function. If so, it cannot be used on a standard Apple II disk port, because there is no way to output a signal on that particular pin. -- David Empson dempson@actrix.gen.nz