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MFM drives in AT machines

1ST1 said:
putting 16 ms in the table accelerates the "full take" about 10 seconds.
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Do you mean "to about 10 seconds"? 0.016 x 615 = 9.84 seconds. So the stepping portion of this entire "blasting everything off the disk" exercise is only 10 seconds. Why not go ahead and assume the worst case of 20ms a step, that takes traversing the whole disk a step at a time to only 12.3 seconds.

So, let's bandy about some other "guess" numbers: with a 1:1 controller you're reading a *track* every 16.7 ms. There are four tracks per cylinder on an ST-225, so if we assume that the controller could just go straight to reading the next track then the total time to read a whole cylinder *plus* stepping to the next is about 87 milliseconds. ((16.7*4) + 20) Multiply by 615 and that's a whopping 54 seconds to read the whole disk.

Even if we assume that the cylinder read time is twice the single track time (IE, there's one rotation's worth of latency when switching tracks within a cylinder, which there really isn't reason to assume if the designers of the controller were smart) the total read/transit time only goes up to around 95 seconds. I'd think if there was that much penalty going from track to track it would have shown up in those benchmarks I ran back in the day on my 1006, though.

(FWIW, a 20MB drive in 54 seconds is "only" about 370Kps, and 95 seconds 210Kps. That latter number is within reach of the original MFM controller used in the IBM AT, interleave and all. So these just don't seem unreasonable to me.)
 
I believe this is history being written here. This will all be forgotten soon as time takes it all away. I'm wanting to keep my Amstrad PC1512SD in working order for prosperity, nothing more. They are becoming museum pieces.
I wish to thank all the commentors above. I was really frustrated until I found this page. Thank the good lord the http links are now https or my goose would be cooked, so to speak. Microsoft deleted all their http links. There is no justice, there's just us.
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Chuck(G) said:
I have a problem with people calling them "MFM drives". The MFM, or any other encoding, is a function of the controller, not the drive. You can use any modulation/encoding that you'd like within the operating parameters of the read/write channels. My 14" SA4008 drive uses MFM encoding, for example. My 700MB 5.25" ESDI drive uses MFM also. Most PC floppy drives use MFM encoding. Neither is apparently what the OP is speaking of.

It's far preferable to identify the drive bt its interface type, as in "ST506 interface" or, better, "ST412 interface". FWIW, I don't believe there was ever a generic name given to the interface; it's always been associated with Al Shugat's Seagate.

It's similar calling the RTC/Configuration RAM device "CMOS". By the time of the 80386, the typical motherboard had lots of CMOS logic ICs.

/rant
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I know this an old post however both the drives and the controller are MFM which is Modified Frequency Modulation and is RLL line code. MFM Drives need the proper MFM Controller card to run. RLL was the same.
 
Sorry, but I'm not following. What is "RLL Line" code?
In fact, both MFM (RLL (1,3)) and RLL (RLL(2,7)) are run-length limited codes. Both commonly-used ST412-interface types have the same demands for bandwidth, but RLL(2,7) requires somewhat more accurate timing, which can be affected by the coating material and read amplifiers. The rest in the controller is mostly a matter of decoding.

In fact, FM (RLL(0,1)) is far more tolerant of timing issues than either of the preceding, but hasn't been seen on hard disks in a very long time, due to its inefficiency (2 bit cell times per bit).
 
staycool72 said:
I know this an old post however both the drives and the controller are MFM which is Modified Frequency Modulation and is RLL line code. MFM Drives need the proper MFM Controller card to run. RLL was the same.
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Uh, you do know that the guy you replied to is a magnetic media recovery specialist, right? Chuck(G) knows of what he speaks here. Search for Chuck Guzis Sydex.

And I agree with his rant. Any encoding can be used to various degrees of success on a ST506/412 interface. The so called RLL drives are functionally identical to the so called MFM drives, but the 'RLL' encoding gets 1.5 times the data on the same physical drive. I used more than one ST225 as a 30MB RLL drive back in the day, as well as ST238R drives (the 'RLL certified' ST225) as 20 MB 'MFM' drives. Other encodings can be used.

The later ST225s worked better than earlier ones, but there were always more bad sectors than using MFM encoding.
 
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