kaleb@mars.jpl.nasa.gov (Kaleb Keithley) (03/15/90)
In article sorc@carina.unm.edu (Paul Caskey) writes: >paravia@csd4.csd.uwm.edu (Mark David Kakatsch) said: > >Mark> Can an MFM drive be connected to an RLL controller? I can get a >Mark> 40Mbyte MFM drive for free, but I've got a '386 w/ an 65Mbyte >Mark> RLL. (Gateway). I know that MFM is pretty slow for a 386, but >Mark> since it would be free, I wouldn't mind...Thanks much. > >Very simply, the answer is: "No, not reliably." IOW, go for it. :-) >RLL is tough on drives. If you have a drive that's made for MFM, but >it's really tough, it may handle RLL. For instance, my roommate >recently got an old DEC Rainbow with a 10 meg MFM drive. That >computer and the drive in it had been sitting in a snow bank for a >week before he bought it. I have a feeling it could probably handle >RLL. > >However, for the average PC drive, chances are that if it was built >tough enough to be an RLL, it *would* be an RLL. Therefore I suspect >that forcing an MFM to do RLL is pretty risky. > RLL is *NOT* tougher on a drive than MFM. RLL uses a different encoding scheme to achieve a higher *data* density, however the physical bit density on the platters is exactly the same. Very old drives that don't hold tight timing tolerences (like older Seagate ST-251s) are more apt to fail at RLL because RLL relies on hardware timing to work, whereas MFM is "self clocking" and doesn't need the precision timing to achieve reliability. The best advice, is try it and see. Perhaps it's worth paying a little extra for an RLL controller with the right to bring it back and get an MFM controller if RLL doesn't work. I have first hand experience with RLLing Seagate ST-225s, newer Seagate ST-251s, and Micropolis 1325s. I once had an older ST-251 that just would not RLL. My experience with the ST-251s would lead me to believe that U.S. made ST-251s won't RLL, while the oriental ST-251s will. This is an *extreme* generalization, based on a sample of two, so probabilities are just not very good that my generalization is a fair one. Others have reported success with other makes and models. To address the original comment about speed, MFM is only slower than RLL (at 1:1 interleave) because for a given revolution, 17 sectors go under the head compared to 26 sectors on RLL. 1:1 RLL reaches xfer rates of about 750,000k(bits or bytes, I can't recall) per second. 1:1 MFM reach xfer rates of about 500,000k. My personal impression is that unless you are doing some very disk intensive stuff, or have a 33mhz 386, or a 25mhz 486, you won't be able to tell the difference between 1:1 MFM and 1:1 RLL. kaleb@mars.jpl.nasa.gov Jet Propeller Labs Kaleb Keithley spelling and grammar flames > /dev/null
RFM@psuvm.psu.edu (03/15/90)
There's been a *Flap* in the mag PC Resource lately about this. A hardware guru there -- Roger Alford -- says Miniscribe 3650's -- 40 meg MFM drives -- will format abhout 80% of time with an RLL driver. He says try it & see; it will or it won't. Bob M, Penn State. ** No Responsibility, etc.**
davidsen@sixhub.UUCP (Wm E. Davidsen Jr) (03/18/90)
In article <3087@jato.Jpl.Nasa.Gov> kaleb@mars.UUCP (Kaleb Keithley) writes: | RLL is *NOT* tougher on a drive than MFM. RLL uses a different encoding | scheme to achieve a higher *data* density, however the physical bit density | on the platters is exactly the same. Well, not exactly. Actually standard MFM is really "RLL 1,3" while most people mean "RLL 2,7" when they say RLL. The first number is the *closest* spacing between flux changes on the media in terms of the data clock rate, while the second is the *greatest* distance. Now even with the data clock running 50% faster, the MFM controller actually puts the flux changes closer together than the RLL. The problem is the second number. Since the RLL controller is waiting a longer period between pulses, the accuracy of rotational speed (usually good) and the read amplifiers (frequently not so good) determines if a drive will work RLL or not. I have yet to see any rational explanation of damage to a drive done by running RLL. I am assurred that the MFM and RLL rated drives from most manufacturers are made on the same lines, and the RLL drives are just tested better. The reason they are not rated RLL may be (a) because they weren't tested, and the manufacturer doesn't want to eat one on warantee if it will run MFM but not RLL, or (b) because it was tested and failed. (a) is more likely, and therefore (read this carefully) most MFM drives for which there is a matching RLL version will run RLL. Of course you have no comeback if the drive won't run RLL, but I have had very good luck with recent Seagates, Priams, etc. I have had some bad luck with Maxtor, which surprised me, and which may not be typical. -- bill davidsen - davidsen@sixhub.uucp (uunet!crdgw1!sixhub!davidsen) sysop *IX BBS and Public Access UNIX moderator of comp.binaries.ibm.pc "Getting old is bad, but it beats the hell out of the alternative" -anon