cyamamot@aludra.usc.edu (Cliff Yamamoto) (04/12/90)
Greetings! This question has probably appeared before, but I have never heard a realistic answer to it. I was going to get a Perstor 180 controller to run ARRL (31 sec/trk) using a ST4096. I've called Perstor and they say it works but when I asked if they *guarantee* my drive will work, they refused. "We can't guarantee the quality of hard disks put out from manufacturers". Ok made sense. But why do they sell controllers like this in the first place? (QUESTION #1). So I called Seagate to see if they had any qualms about it. The guy said he's heard of people running MFM drives at higher capacity, but Seagate doesn't recommend it (ok.. sounds familiar). I asked what's the difference between MFM and RLL? He tells me just the electronics since data rates and density are different. I ask him about media differences and he says both MFM and RLL use the same type of media nowadays. Well can't the analog portions of the disk electronics be "retuned" to run MFM, RLL, ARLL or ESDI? (QUESTION #2). Well I could have saved about $330 going the Perstor way (180-16FN with ST4096). But since I've not been convinced that ARLL can be *safely* run on a MFM drive, I went out and ordered a WD10007-SE2 with a 1355 Micropolis drive. I'm sure I should be 'more safe' with this ESDI combination. Yet I'm still curious; how does "tuning" the electronics for a particular drive affect the magnetic patterns on the disk? (QUESTION #3). Does the read/write head require more current to generate higher flux? (QUESTION #4). Or is even the physical geometry of the read/write head on a MFM vs. RLL entirely different? (QUESTION #5). Do the amplifiers on the analog board need to be of higher gain for different encoding formats or is just higher bandwidth necessary? (QUESTION #6). This turned out longer than I wanted but I since I shelled out $330 more, I want to feel "justified" that there is some legitimate technical reason why higher capacity drives cost more even though others may be doing the same with their MFM drives (at a greater risk??!?) Please reply via email or post to these newsgroups (I hope others may be interested in this info as well). Clifford Yamamoto - KA6JRG Email: cyamamot@aludra.usc.edu Jet Propulsion Labs cyamamot@devvax.jpl.nasa.gov 4800 Oak Grove Drive cyamamot@jato.jpl.nasa.gov M/S 238-737 cky@hydra.jpl.nasa.gov Pasadena, CA. 91109 (818) 354-1242
scjones@sdrc.UUCP (Larry Jones) (04/13/90)
In article <9038@chaph.usc.edu>, cyamamot@aludra.usc.edu (Cliff Yamamoto) writes: > I was going to get a Perstor 180 controller to run ARRL (31 sec/trk) > using a ST4096. I've called Perstor and they say it works but when I > asked if they *guarantee* my drive will work, they refused. "We can't > guarantee the quality of hard disks put out from manufacturers". Ok > made sense. But why do they sell controllers like this in the first > place? (QUESTION #1). Why do people sell RLL controllers? Because it's a way to get more data on a drive, thus reducing the cost per bit of storage. If it works, it's great. If it doesn't, it's too bad. I'm not surprised they don't guarantee that it will work, but they do guarantee to let you return the controller and get your money back if it doesn't. > So I called Seagate to see if they had any qualms about it. The guy > said he's heard of people running MFM drives at higher capacity, but > Seagate doesn't recommend it (ok.. sounds familiar). I asked what's > the difference between MFM and RLL? He tells me just the electronics > since data rates and density are different. I ask him about media > differences and he says both MFM and RLL use the same type of media > nowadays. Well can't the analog portions of the disk electronics be > "retuned" to run MFM, RLL, ARLL or ESDI? (QUESTION #2). ESDI is a disk interface, not a record technology like MFM, RLL, and ARLL. The analog electronics do not have to be different, it's just that RLL and ARLL require tighter tolerances than MFM does. In order to fit more data on the disk, RLL puts the bits closer together. When it reads the data back, it needs to know the positions of the bits more accurately. ARLL only puts the bits a tiny bit closer together, but needs to know the position much more accurately when read back. Since this is somewhat error prone, ARLL also adds a very long ECC (Error Correcting Code) to detect and correct longer error bursts than MFM or RLL usually do. So, if the drive electronics are fairly stable, ARLL will work fine where RLL won't. > Well I could have saved about $330 going the Perstor way (180-16FN > with ST4096). But since I've not been convinced that ARLL can be > *safely* run on a MFM drive, I went out and ordered a WD10007-SE2 > with a 1355 Micropolis drive. I'm sure I should be 'more safe' with > this ESDI combination. ESDI is safer because ALL of the analog electronics are on the drive itself rather than on the controller. Since the low-level bits don't have as far to go, there is much less chance of them getting messed up or shifting around. Thus, the required position sensing is fairly easy. > Yet I'm still curious; how does "tuning" the > electronics for a particular drive affect the magnetic patterns on > the disk? (QUESTION #3). Not at all. > Does the read/write head require more > current to generate higher flux? (QUESTION #4). No. > Or is even the > physical geometry of the read/write head on a MFM vs. RLL entirely > different? (QUESTION #5). No. > Do the amplifiers on the analog board > need to be of higher gain for different encoding formats or is > just higher bandwidth necessary? (QUESTION #6). Higher bandwidth is needed for RLL, less so for ARLL, but higher stability is required for ARLL, less so for RLL. ---- Larry Jones UUCP: uunet!sdrc!scjones SDRC scjones@SDRC.UU.NET 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 "You know how Einstein got bad grades as a kid? Well MINE are even WORSE!" -Calvin