RL02K Pack Reliability/Characteristics

Johnny Billquist bqt at update.uu.se
Mon Mar 30 22:35:01 CDT 2015

On 2015-03-31 01:50, Christopher Parish wrote:
>>> Were all of the packs written on systems with the same word length? If come
>>> were written on a PDP8-A, You might get garbage reading them on a PDP11.
>> For some definition of garbage, sure. But it would be the same garbage every time.
>> Johnny
> That was my thought as well, even if I was dealing with 12-bit data word size packs, I should always get the same data every read out


>> Christopher, have you tried more than one drive?
>> Pete
> Yep. The effect seems somewhat random, which is why I suspect a problem with the controller, although I can't rule the drives out.

Seems more likely to be the controller. But we shouldn't rule anything 
out. :-)


> 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.
> Without a PDP-11 to checkout the drives and packs, it's no fun not having a known good configuration to start from. :(

I assume you are aware of that any software driver for the RL drives 
needs to read the headers to verify that they are on the right track, 
after doing a head select (or rather a change in which head you have 
selected), and do a new seek if on the wrong track. Repeat until on the 
right track.

Essentially, seeks on the RL drives are not reliable, and you must 
always first check that you are on the right track before doing anything 

I would almost suspect your controller is just fine in this aspect. 
(This is documented in manuals, if you read enough.)


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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