Talk:ST506/ST412

message: The ST506 and ST412 manuals that I have looked at (through bitsavers)
indicate that the drives were Seagate.  
It looks to me that Shugart drives were numbered with SA,
and Seagate with ST, further evidence for Seagate.
I don't know when the change was made, though.
Gah4 (talk) 21:27, 17 January 2008 (UTC)
replies: text: Shugart Associates [f. 1973], later Shugart Corporation is a different company than Seagate Technology originally  Shugart Technology [f. 1979].  Alan Shugart was a founder of both. Tom94022 (talk) 17:00, 18 January 2008 (UTC)

text: Seagate Technology Tom94022 (talk) 06:08, 14 February 2008 (UTC)
thread_title:  Seagate or Shugart? 

message: Out of curiosity, how much did one of these cost in 1980? I imagine they weren't available to the public, and so researching the cost would be hard, but perhaps someone reading this was actually there, in 1980, and can provide a source. -Ashley Pomeroy (talk) 23:21, 1 January 2011 (UTC)
replies: text: The Seagate Sept 24, 1981 Prospectus gives the FY1981 revenue of $9.9 million for approximately 13,000 drives or an ASP of about $750.  The claim then was that this was the first HDD priced below $1000 which probably was true although the Shugart SA1000 or the Memorex 101 may have beaten it below that threshold.  Tom94022 (talk) 00:07, 2 January 2011 (UTC)
thread_title:  Cost 

message: The interface is limited in number of heads because the 4-bit head addressing can only express 16 combinations. The drive selection scheme is one wire per drive, so only 4 drives. And controllers only expect 5 Mbit/s per documentation. Bytesock (talk) 13:08, 28 April 2016 (UTC)
thread_title:  Interface limitations 

message: The paragraph was reverted because it ascribed as a "limitation" a mixture of some specifications and some unsupported limitations.  The ST506 interface is indeed limited by the 4 drives select and 3 or 4 head select lines.  If used according to the Seagate specification it can address either 8 or 16 heads, but one can use unused drive select lines as head bits going beyond 16 so the limitation is not 16.  I know of know published 6.1 meter cable length limitation and it is not supported by the citation.  The data rate is clearly not limited to 5 Mbits/sec since it is capable of 7.5 Mbits/sec when used in RLL and most likely even more since the original 5 MHz wasn't picked from any transmission limitation.  Acurately written the sentence would be OR and since it didn't add a lot to the article, I reverted.  Tom94022 (talk) 21:12, 28 April 2016 (UTC)
replies: text: The specification does indeed limit what you can do with any interface.  Of course, the interface can be changed circumventing the limitation, but then it would no longer be a ST-506 interface which is what the article is about.   You clearly did not read the specification cited because the maximum cable length is specified as 20 feet (which is 6.096 metres - but 6.1 metres is close enough).  A relatively simple calculation gives the theoretical maximum length as 6.9 metres so 0.8 metres is a reasonable safety margin. --Elektrik Fanne 16:28, 26 July 2016 (UTC)
replies: text: The drive is designed around 5 Mb/s MFM, to satisfy bit error rate specifications. As with processor overclocking, one can use things out of specification. An important limitation is flux change density, and the RLL controller conveniently stores more bits with the same maximum flux change rate, but also has stricter requirements on flux change timing. I do remember much discussion on which drives were specified to run on RLL controllers, and which weren't, even if there wasn't any actual difference. Gah4 (talk) 03:49, 1 June 2018 (UTC)
replies: text: I think it is more about margin than flux change rate or density.  The bit detection window is 33% smaller in RLL than in MFM (hence 1.5 x capacity and data rate). Most drives (particularly stepper motor drives) shipped with a lot of window margin to allow for off track due to thermal and mechanical changes, so operating in home or office environment most ST 412  MFM drives would appear to initially work on RLL controllers but over time would have increased soft and hard error rates. A drive left on and working in an air-conditioned office might have a bunch of trouble when the air-conditioning failed. I think some vendors just selected their high margin drives and labeled them as RLL qualified but for high yield production the vendors had to improve the margin by doing things like changing formulation, flying lower, reducing off track, etc.  Tom94022 (talk) 19:17, 1 June 2018 (UTC)
thread_title:  Limitations vs specifications 

message: It is true that the longer seeks take longer because in part they require more step pulses.  But is "The number of tracks increased the access time." as was once stated in the article true?  The reference spec has the same "average access" times for the 612 track ST506 and 1224 cylinder ST412 so on its face in the reference the answer is no.  A better question would be does the average seek time increase as the number of cylinders increases and which also on its face appears to be no.  A more subtle answer would note that each additional cylinder requires one additional step pulse which in burst mode would require at least 25 more microseconds so adding 100 cylinders might add 2.5 msec.  But some vendors measure average seek time from the last seek pulse in a burst so in that mode and with that definition there is no added time.  And vendors of drives compatible with the ST412 interface but having more cylinders take into account the additional step pulses necessary for their additional cylinders.  So there maybe no real world example where additional cylinders increase average seek time.  On the whole I suggest the article is better off without this subtle distinction, if there is one at all.  Tom94022 (talk) 18:18, 31 May 2018 (UTC)
replies: text: It's generally true that more tracks increase seek time with stepper-actuated drives. However, the ST-412 introduced a 'burst' mode for stepping (note the manual's ** footnote) that was supposed to reduce the average seek time to that of the ST-506 – the burst stepping mode didn't really catch on (it required controller support), so in reality, the access time increased. The manual is not a good source on this but I don't have a better one. I don't insist on including the statement in the article, we can well leave it out. --Zac67 (talk) 19:55, 31 May 2018 (UTC)
replies: text: *I think you are referring to mainly FDD interfaces which only operated in a "slow" mode.  The ST-506 operated that way, but pretty soon afterward the HDD industry adopted the "burst" mode.  The Shugart Associates SA-1000 which preceeded the ST-506 and upon which the ST-506 interface is based provided a burst mode.  I think Seagate left out the logic to savea a few dollars.  Tom94022 (talk) 22:29, 31 May 2018 (UTC)

text: I haven't thought about this for a long time. Do all drives buffer seek pulses, and then run the stepper motor at the appropriate rate, or directly run the stepper, like floppy drives? Since the distance moved is the same, it seems somewhat reasonable that the time is about the same. (Inertia, and all that.) There is then settling time.  Reminds me that wikipedia likes secondary sources over primary, but I suspect any secondary sources just copied the primary. Gah4 (talk) 19:59, 31 May 2018 (UTC)

text: OK, from ST412 OEM manual, which also describes the ST506, the seek times, using the appropriate for each optimal step times, are the same. For ST506, you cut a jumper, and then vary the step times for optimal motor operation. For ST412, seeks are buffered by the drive, if sent faster than 200us. Or you can do slow (3ms) step speed on either one. Reminds me around 1989, I had a program to measure seek time for MS-DOS. I ran it on an actual IBM machine, and the message was something like "This disk is so slow, are you sure it isn't a floppy drive?"  I have no idea which controllers did the ST506 fast seek algorithm.  Or maybe it is done in software?  Gah4 (talk) 20:38, 31 May 2018 (UTC)
replies: text: I too haven't thought about this in a long time, but it is my recollection that burst mode was standard on all drives complying with the ST412 interface "standard"  and that all corresponding controllers then burst at the maximum rate.  And as drive manufacturers introduced new models with more cylinders in the same recording space they sped up the mechanisms so the average seek time got lower as tracks were added in the same space. Thus the ST251 with a ST412 interface and 820 cylinders has an average seek time of 40 msec. The article can probably be improved by noting the ST412 interface became the industry standard with a few changes such as the busrt mode standard.  Tom94022 (talk) 22:29, 31 May 2018 (UTC)
replies: text: Yes, it seems that burst mode came with ST412, such that the drive figures out the optimal step rate. The ST506 service manual mentions that it stops with two phases powered for faster settling, and so two steps per track. But ST506 has algorithm driven seek, though I didn't look up the algorithm. The controller has to vary the step rate following the algorithm. My first hard drive is (I think I still have it) a Miniscribe 6053, with I believe (after many years) a voice coil positioner.  No stepper motor to limit the speed. It looks like with optimal controller the seek times are the same. That doesn't mean that such controllers were common. Gah4 (talk) 02:15, 1 June 2018 (UTC)
replies: text: Since the reference source mentions algorithm driven seek, which can be much faster than fixed step rate, the article should mention that in the seek rate comparison. I don't know if the ST001 technical note is available, though, with the actual algorithm. Gah4 (talk) 21:45, 1 June 2018 (UTC)
thread_title:  Average seek time 

message: The discussion about timing windows and RLL reminds me that there isn't mention of precompensation. This was usual for floppies, and extended to ST506 and such, and is described in the references. Gah4 (talk) 21:49, 1 June 2018 (UTC)
replies: text: Actually it is sort of coverred in the article in the change in definition of the pin 2 from Reduce Write Current to Hd Sel 3 - another of the 506 -> 412 evolutionary changes and probably not worth elaborating on in the article.  BTW  did u notice we have been talking about this article off and on for ten years? Tom94022 (talk) 22:57, 1 June 2018 (UTC)
replies: text: No, I hadn't thought about it being ten years.  It seems to have been my 18th edit, and over a year after my first edit. Gah4 (talk) 23:11, 1 June 2018 (UTC)

text: It seems that there is a Write precompensation, but it gives much less detail that it probably should. Gah4 (talk) 23:11, 1 June 2018 (UTC)
thread_title:  precompensation