Path: utzoo!attcan!uunet!lll-winken!scooter!neoucom!wtm From: wtm@neoucom.UUCP (Bill Mayhew) Newsgroups: comp.sys.att Subject: Re: 3B1<->3B2 Summary: Specifications of 1.2 megabyte disk drive Keywords: what compatibilities? Message-ID: <1447@neoucom.UUCP> Date: 5 Jan 89 23:26:56 GMT References: <7047@ihlpf.ATT.COM> <598@whizz.uucp> <537@mccc.UUCP> <601@whizz.uucp> Organization: Northeastern Ohio Universities College of Medicine Lines: 56 Threre are many things that cause incompatibility between 360K 40 track drives and 1.2 megabyte 80 track drives. Most notable is the fact that the media in 1.2 meg drives must have a higher MOL (max- imum output level) to compensate for the approximate 3 dB loss of signal from the disk head (the tracks are half the width, hence approximately half the amplitude is recovered in a read of an 80 track drive). To accomplish this, 1.2 meg diskettes use a cobalt doped oxide (to the eye, it looks black, rather than brown of 360K disks). This is akin to chrome cassette tapes. Just for the record, a 360K drive can not generate a field intense enough to magnetize an HD type diskette. The devices are thus incompatible at the media level. A 1.2 meg HD disk drive can, however, read a 360K diskette. The converese is not true; an HD drive can not write in low density such that reliable reads are guaranteed on a 360K drive. This makes it possible to port from a 360K machine up to a 1.2 mege machine, but not reliably vice versa. An HD disk is laid out thus: 80 tracks * 2 sides * 15 sectors per track * 1/2 Kbyte per sector = 1,200 Kbytes per disk. To get decent MOL, the RPM is also increased on a 1.2 meg drive from 300 pm up to 360 rpm. This gives a data transfer rate of 500,000 bits per second of raw data to/from the head. A 360K disk is liad out thus: 40 tracks * 2 sides * 9 sectors per track * 1/2 Kbyte per sector = 360 Kbytes per disk. The drive runs at 300 rpm, which yields 250,000 bits per second measured at the head. (IBM type MFM encoding assumed.) The Unix PC can also be programmed to format at 10 sectors per track. A 720K quad density drive is much like a 360K drive, except that there are 80 tracks of 1/2 the normal width. The oxide used is approximately (if not exactly) the same as that used in 360K media. To the best of my knowledge there aren't any readily available HD 1.2 meg drives that can read the oddball 720K format. The best bet would be to attempt to teach cpio (if it doesn't already know) how to read the 3b1 diskettes on the 3b2. You might be able to go 3b2 --> 3b1, if you use virgin (or bulk erased) media on the 3b2 and then only read, but never wrtie to it on the 3b1. The key is to only ever write on the disk (3b1 or 3b1) with the machine that did the actual formatting. There is a company that makes an oddball 1.2 meg drive that runs at 300 rpm for use in XT compatible machines that have FDC chips that are fixed at 250,000 bps. Priority One Electronics either does or did stock such drives. I have never used one of those beasts, so I don't know how well it works. I can't vouch for the quality of Priority One either, as I have never bought anything from them. Priority One usually advertises somewhere in Byte each month. --Bill