RL02K Pack Reliability/Characteristics

[Mike Ross]

That's a very interesting project. Do you have plans to make this
thing available in some form once the bugs are out?

It's high time someone collated all the 'new hardware' projects out
there - this, Vince's Omnibus memory boards, the widget drive
replacements for Lisa, Guy Sotomayor's stuff, all these things -
should be a one-stop shop where info on all this stuff is held. Rich
Alderson, I know you guys have done work towards plug-compatible
pdp-10 memory & disk replacements, do you think that could ever be
shared? I'd love to see a Massbus connector with a CF or SD card
slot... :-)

Maybe I'll have to take up the job of collating these projects :-)

On Tue, Mar 31, 2015 at 4:41 PM, Johnny Billquist <bqt at update.uu.se> wrote:
> On 2015-03-31 01:57, Paul Koning wrote:
>>
>>
>>> On Mar 30, 2015, at 7:50 PM, Christopher Parish
>>> <christopher.parish at parishcomputers.com> wrote:
>>>
>>> ...
>>> Second, I've noticed that the drive seems to mis-seek on occasion.  I
>>> command the drive to walk forward or back a single track, and the heads move
>>> but sometimes land on the same starting track.  This results in duplicate
>>> data for the next 10kB or so, and is heavily pack dependent.  Some packs
>>> don't exhibit it, others do.  I imagine this is related to the drive/pack
>>> runout condition described in the manual.  Until I figure out more, my plan
>>> is to add additional verification to the incoming data, checking the track
>>> it landed on and re-commanding the difference if necessary.
>>
>>
>> As I recall, RL01/02 have embedded servo (one of the first DEC drives to
>> do so).  That would suggest you have a marginal servo mechanism.  As for
>> wrong data, isn’t there a track (cylinder) number in the headers?  I would
>> expect there to be one, so a wrong seek should be detectable.
>
>
> Right, right and right. Except I don't know if the servo mechanism
> necessarily is marginal. The RL drives are "funny" in that there aren't even
> any absolute seeks. When you seek on those drives you simply tell it "seek n
> tracks in or out". The value of "n" is something you need to calculate based
> on what your current track is, and what track you want to get to. And when
> the seek is done, you repeat if you detect that you are not on the right
> track. The drive itself have absolutely no checking if it got to the right
> track or not, since it don't even know what the "right" track is.
>
> And a switch of head select does an implicit seek to whatever track is
> closest to the other heads, which means a head switch can cause the drive to
> move to a different track. So you always need to do a seek cycle after a
> head switch.
> While disks would be formatted with the tracks on both side of the platter
> aligned, there is no guarantee that the heads in the drive are actually
> perfectly aligned, which is why you get the head movement on head switch.
>
>
>         Johnny
>
> --
> Johnny Billquist                  || "I'm on a bus
>                                   ||  on a psychedelic trip
> email: bqt at softjar.se             ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol



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