RL02K Pack Reliability/Characteristics

Johnny Billquist bqt at update.uu.se
Mon Mar 30 22:41:20 CDT 2015

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 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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