[Info-vax] TK50 - this is annoying...

[Paul Sture]

In article <507fb982$0$52259$815e3792 at news.qwest.net>,
 George Cornelius <cornelius at eisner.decus.org> wrote:

> Paul Sture wrote:
> 
> > In article <QMHlFQS6wZyr at eisner.encompasserve.org>,
> >  koehler at eisner.nospam.encompasserve.org (Bob Koehler) wrote:
> > 
> >> In article <k4v41o$bte$1 at Iltempo.Update.UU.SE>, Johnny Billquist
> >> <bqt at softjar.se> writes:
> >> > 
> >> > Even by asking that question, you reveal that you have not used the
> >> > TU58... :-)
> >> > Slower than DECtape? Hell yes. DECtape flies compared to it. DEC did
> >> > call the TU58 for DECtape II, but that really an abuse of the DECtape 
> >> > name.
> >> 
> >>    You got that backwards.  I've done a standalone backup boot off TU58,
> >>    later followed by an OS upgrade.
> >> 
> >>    I've seen, but not used, DECtape.  I had no idea how fast it actually
> >>    moved the small amount of data it held.
> > 
> > I once met someone who had used DECtapes and he had been very impressed
> > by them in their day, but he was at least 20 years my senior.  IIRC he
> > described some mechanism where they skipped alternate blocks when
> > reading or writing so that the tape speed could be higher, and those
> > skipped blocks were used when the tape was travelling in the opposite
> > direction.
> 
> I had heard that the data was written twice and always assumed (I
> suppose correctly) that the extra blocks were for redundancy.  That
> they might have been written in reverse bit order never occurred to
> me.
> 
> I used them with Dibol on a PDP8/I and thought they were very slow
> on write.  Then we got in the PS/8 (predecessor to OS/8?) O/S and
> I found out otherwise.  Apparently DIBOL was writing with verify.

In my RT-11 + DIBOL days an auditor asked whether write verification was 
done.  I satisfied him by pointing at an assembler example in the 
manuals which showed it being done and he was happy :-)
 
What I didn't realise until I read the DECtape Wiki entry was how 
unreliable early hard disks were.  By the time I entered the computing 
world (in 1977) disks were reliable enough to store data on. The PDP 
11/34 I worked on from 1978 had disks only - no tape, card or paper tape.

> They were quite fast from my point of view at the time, and I
> remember them being reliable.  At 200 bpi, with redundant writes,
> on 1/2 inch reel-to-reel media, why not?

We didn't fling so much data around in those days either :-)

-- 
Paul Sture