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

[Paul Sture]

In article <k523mt$cti$1 at speranza.aioe.org>,
 glen herrmannsfeldt <gah at ugcs.caltech.edu> wrote:

> Johnny Billquist <bqt at softjar.se> wrote:
> > On 2012-10-09 20:49, glen herrmannsfeldt wrote:
> >> Paul Sture <nospam at sture.ch> wrote:
> 
>  (snip)
> 
> >>> 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 haven't heard about that but, just like floppy disks, I suppose
> >> you could do block interleaving. As well as I understand it, DECtape
> >> is direct access in the same way floppy disks are, with block headers
> >> identifying the block number.
> 
> > Correct.
> > But I too have not heard of anyone actually doing this kind of 
> > semi-interleaving. Most machines I've seen or used don't have a problem 
> > keeping the DECtape running at full speed across several blocks.
> 
> It would seem to require lower-level access to the drive than usual,
> at least for the read backwards part. Forward, it should just look
> for block headers.
> 
> > But you could definitely do it. But unless the controller managed all 
> > the reverse twiddling required, it hardly pays to read in reverse. 
> > You'll get the words in the wrong order, meaning you need to DMA from 
> > last address to first. 
> 
> But if you write in reverse, then the bits should come out right.
> That assumes that the motor supports read/write speed both directions,
> instead of just rewind in reverse.

That's how I understand it too.  According to the Wiki there were 
various controllers for DECtapes, some more sophisticated than others. 
It is possible that the method described to me was only present on the 
more sophisticated variety.

-- 
Paul Sture