Yet another idea for recovering the data from Series80 tapes

[Tony Duell]

> 
> ard at p850ug1.demon.co.uk (Tony Duell) wrote:...
> > Hmmm. I thought it was where the taps stuch to itself, or to the drive
> > belt, when the catridge was stored. In which case your idea might not
> > help much.
> 
> Aha! There goes my original hypothesis. Well, that is relatively easy to
> check. I took apart two old tapes and found the following:
> 
> Assuming that the tapes were rewound before storage, there are two
> stretches of weakened magnetic coating on both tapes I looked at; the first
> was short while the second longer. These correspond to the length of tape
> in contact with the drive belt

This would seemm to imply my guess was correct _in this case_. Of course 
other tapes might fail in other ways.

> Using a marker to paint (on the back side of the tape) over the region
> does not work, since the markers I tried allow some light to go through.
> I stuck a piece of splicing tape and that took care of the hole, but I
> got a DATA error. I guess the 85 needs some of the info that is missing
> from the tape.

Of course if you did the trick you were suggesting and read the tape at 
the flux-transition level, you could at least try to recover the data 
that was on the undamaged tape. It might be possible to piece together 
some of the files.

I assume the tape format is somewhat LIF-like. That is, there's a single 
directory at the start of the tape, and files are contiguous. 
 
> So the new plan is to see how I can get the tape moving at 10 ips
> (I'll look at the 9815 diagrams) and see how I can tap the output of

I think the 9825 controller is easier to understand (and easier to use 
outside the machine if you mamange to find one). 

> the read/write IC (1820-2418) to get the bits from the tape.

I would not try to use that IC. AFAIK it's a single-channel device, you 
can't read both heads simultaneously. Look at the 9815/9825 schematics to 
get an idea how to build a read amplifier using standard parts. You'd 
need to build 2 of them of course, one for each track.


> ard at p850ug1.demon.co.uk (Tony Duell) wrote:
> > Does anybody know if those drives are tape and track compatible
> > with the 85 drive? In other words, would the 2 track head of a 9815
> > correctly read the tracks on an 85 tape (yes, I know the controllers
> > are very different).
> There was a program on the Series-80 User's Library (8291029) that
> allowed retrieving 9845A SAVEd programs. The program was for the
> HP-85 and did not specify any ROMs or peripherals. You may infer
> from that, that the Series 80 could read 9845 tapes.  On the other
> hand, the 9845 used dual directories (the second one was a backup)
> per tape while the HP85 only uses one, so this may be an indication
> that they are NOT compatible.

I know for a fact that the 9845 tape drive is the same as that in the 
9815 and 9825. 

I think the data format is very different, but the drive is compatible in 
the snese that the track positions are the same, etc (if not, the 85 
wouldn't have a hope of reading the 9845 tape). Which means you might be 
able to do the reverse.

I would not try to use a 9845 tape controller. There's a big, heatsinked, 
HP custom chip in the middle, linked to a DAC for the motor speed 
control. The servo loop, read/write circuit, etc, is all partly inside 
that ASIC. And it's not even software compatible with the 9825 (it uses 
different lines of the I/O bus, IIRC the 9825 uses DMA, the 9845 doesn't, 
etc).

-tony