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

[Johnny Billquist]

On 2012-10-18 16:44, glen herrmannsfeldt wrote:
> George Cornelius <cornelius at eisner.decus.org> wrote:
>
> (snip, someone wrote)
>
>>> 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.
>
> The data is written twice, in parallel. Three data bits on 10 tracks.

I think it's time we kill the just created myth of blocks written 
backwards, and what not. That has, as far as I know, never been done.

Yes, data is written twice on the tape, on separate tracks, just like 
you write, Glen.
And that is a big reason why data is rather safe on DECtape. You can 
even punch holes in the tape, and it will still work.

Writing blocks backward would be more headaches that it would be worth, 
not to mentioning, as I did before, that computers can keep the tape 
spinning without missing blocks, so there is no need from an 
optimization point of view.

If people actually thought about it... I mean, a PDP-11 could keep a 
disk fed with data without missing blocks, and disk blocks pass by much 
faster than tapes...
(Heck, I could probably keep a disk fed data on a PDP-8 as well, if I 
thought about it.)

	Johnny