[Info-vax] DECtape (was: TK50 - this is annoying...)

[Johnny Billquist]

On 2012-10-10 12:44, Paul Sture wrote:
> In article <k523mt$cti$1 at speranza.aioe.org>,
>   glen herrmannsfeldt <gah at ugcs.caltech.edu> wrote:
>
>> 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.

It totally depends on the controller.
DECtape (per design) supports reading and writing in both directions.
Exactly how the data will appear to you, in memory, when read in the 
opposite direction to how it was written, depends on the controller. I 
have a TD8E on my PDP-8, and the data will come out inverted in all kind 
of ways. But I won't deny that a TD8E is the dumbest thing made since 
the plain resistor.
And no, the TD8E don't do DMA. It won't even interrupt. It expects you 
to poll. For every bit.

	Johnny