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

[Johnny Billquist]

On 2012-10-18 22:09, George Cornelius wrote:
> Johnny Billquist wrote:
>
>> On 2012-10-18 16:44, glen herrmannsfeldt wrote:
>
> [...]
>
>>> 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.
>
> Actually, my error was thinking that it was separate blocks. And
> I knew better, but those particular bits in my grey matter are
> fading almost as fast as the ones on the DECtape reel I saved
> from my PDP-8 days.
>
> I was surmising that the data bits might have been flipped in
> software at write time, but still writing forward.  Slows down
> writes but improves performance on read reverse.  Just trying to
> make sense of the comment earlier in the thread which, as you
> say, did not accurately describe the situation.
>
> Thanks for setting us straight.

That was mostly Glen. DECtape have 10 tracks. It's one timing mark, one 
control track (can't remember the correct name right now), and three 
data tracks. And then all of these tracks done twice.

But reading the tape backwards means all 0 becomes 1, all data are 
reversed in blocks of 3 bits, and you'll have to do calculations to even 
figure out what the block number values are, as they too gets inverted 
and flipped.

Either do it all forward, or all reverse, is what I can say... Unless 
your controller can do all the flipping for you, in which case you might 
do it in either direction all the time.

	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