30 Mar
2015
30 Mar
'15
8:35 p.m.
On 2015-03-31 01:50, Christopher Parish wrote:
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
Right.
Seems more likely to be the controller. But we shouldn't rule anything
out. :-)
[...]
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 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. 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
else.
I would almost suspect your controller is just fine in this aspect.
(This is documented in manuals, if you read enough.)
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