Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!orion.oac.uci.edu!ucivax!megatek!cjp From: cjp@megatek.UUCP (Christopher J. Pikus) Newsgroups: sci.electronics Subject: Re: What is "ST-506"? Message-ID: <846@valhalla.megatek.uucp> Date: 5 Dec 90 03:17:46 GMT References: <12249@milton.u.washington.edu> Distribution: usa Organization: Megatek Corporation, San Diego, Ca. Lines: 105 Now for a bit of historical trivia: The name ST-506 came from one of the first mass produced hard disks in the 5.25 format: the Seagate ST-506 5 meg drive. The interface between the disk and the controller was pretty much standardized by everyone in the IBM PC business, and is still in use today. In article <12166@milton.u.washington.edu> wiml@milton.u.washington.edu ------- (William Lewis) writes: > > MFM and RLL refer to the format of the data on the disk between the >"1000101010" level and the "north-south-south-north" level. I think most >drives are MFM; > >RLL can get you more formatted capacity for the same drive >but requires higher quality oxide. Some people say you can format an MFM disk >as RLL and reap the rewards; some say you're just asking for trouble. RLL >stands for 'run-length limited'. > From article <12249@milton.u.washington.edu>, by whit@milton.u.washington.edu ------- (John Whitmore): > Not exactly; the drive CONTROLLER determines the MFM or RLL > encoding/decoding. Drives have only low-level functions, not MFM/RLL > encoding and decoding. > > Some drive manufacturers make drives with different preamplifier > bandpasses optimized for RLL or MFM, and the signal/noise coming > from such drives is optimized for one or the other. If the noise > isn't too much of a problem (as for most low-performance drives) > the drive will work with either encoding. > Using a drive with RLL encoding requires reformatting it with > an RLL drive controller board. This is often done on old MFM > drives, and increases their effective capacity. Reliability of > this technique is possibly not the best. > Both of the above persons are correct but for different linguistic reasons. Over the interface between the disk and controller the bits on the disk are transmitted as an analog stream. When reading/writing the disk, the head positioning/control signals are digital but the data con- sists of analog voltage swings above and below 0V; looks a lot like a sine wave. The data separator (thing that converts these zero crossings into clocks and bits) resides on the controller. Since the speed of the drive can vary a bit, the bits have a clock superimposed on them to tell the data sepatator where the bits start and stop. In the MFM (Modified Frequency Modulation) they have one zero crossing for a "0" bit and two zero crossings for a "1" bit. EXAMPLE OF MFM: ---- ---- ------- ---- ------- ---- | | | | | | | | | | | | | | | | | | | | | | | | ---- ---- ---- ------- ---- ---- --- | "1" | "1" | "0" | "0" | "1" | "0" | "1" | As you can see there is at lease one zero crossing for each bit for the PLL to derive the clock from. If you only had transitions on the "1" bit and no transitions on a "0" you could get away with a lot less transitions (1 per bit). The problem is that if you get a lot of "0"s in a row the PLL would fall out of sync. An answer to this is called "bit stuffing" and is used in some synchronous serial protocols (SDLC/HDLC). Whenever you get a certain number (5 in SDLC) of consecutive "0"s ("1"s in SDLC), you insert a "1" ("0") no matter what the next bit is. This way you are putting a limit on the maximum time between transitions. Hence RLL (Run Length Limited). EXAMPLE OF RLL: ---- ------------------------------------- ------- | | | | | | | | | | ------- ------- ---- | "1" | "1" | "0" | "0" | "0" | "0" | "0" |"SB" | "1" | ^ | Stuffed bit ------------+ The data separator will remove the stuffed bit when reading. Since there are fewer zero crossings (north-south transitions) you can put more bits on the disk for a given area. This assumes that the disk is designed for RLL (see comment about bandpass filters above). NOTE: since the data separator for an ST-506 drive is on the con- troller, they are a matched set; once you format a drive, you must keep that drive and controller together until you format it again. To do otherwise is asking for trouble. -- That is all ... Christopher J. Pikus, Megatek Corp. INTERNET: cjp@megatek.uucp San Diego, CA UUCP: uunet!megatek!cjp or ucsd!megatek!cjp Brought to you by Super Global Mega Corp .com