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HP Forum Archive 15

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Recovering the data from Series80 tapes
Message #1 Posted by Vassilis Prevelakis on 5 Nov 2005, 5:40 a.m.

The following is a summary of a thread on the classic computer mailing list.

The problem with the old HP 9162-0061 data cartridges is that the magnetic coating peels off from the (transparent) tape causing the machine to stop accessing the tape with an EOT warning. This is actually very good news because it means that the damage is localized (a few mm of tape with scrapped off magnetic coating).

BTW here is a picture of a damaged tape:

Tony Duell suggested that tapes stuck onto themselves and 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


you can see that on the take up reel (left) there is less tape in contact with the drive tensioning belt, while on the right hand reel (which has most of the tape and hence a longer radius) more tape comes in contact with the belt.

Looking closely at the belt, I can see remnants of tape oxide stuck on it (in fact its stuck so well, I could not remove it).

So how come we get an EOT error when using these tapes?

The tape has tiny holes near the beginning and end signifying BOT and EOT.

      (BOT)                                                     (EOT)
 +-------------------------- .... --------------------------------------+
 |                                                                      |
 |     o o         o                                    o         o     |
 |                                                                      |
 +-------------------------- .... --------------------------------------+

I assume that the first hole signifies the end of the data region and then if the drive sees one hole its EOT, while if it sees two holes (prob. more than one) its BOT.

So when it sees the blank region, it interprets it as one hole and reports end of tape.

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.

I have also noticed that the tape does indeed stick onto itself on the right hand reel, but if you unwind it slowly you can get it unstuck without damaging it. I assume that conditioning the tape by warming it up before use may reduce this sticking problem, but I still would not want to fast wind a tape in that condition.

Given the above, I am even more confident about data recovery, as we may be able to use the existing tape mechanism of the 85 to read the data by skipping the regions that appear blank until the tape un-spools and the drive stalls.

Proposal 1

To do this I would need to manually rewind the tape till the BOT sensor, mount it on the tape drive and advance it past the read/write head till I run out of tape (tape unspools from the supply reel).

While this is going on I am recording the output of both heads via the audio in port of my PC sound card (maybe use two separate sound cards to avoid cross talk?).

Data extraction rig:

1) audio connection: what kind of set-up would be required to connect the heads to the audio card. From the schematics I see that there are essentially three connections to each head:

        Head 0: H0, CT0, and HC
        Head 1: H1, CT1, and HC (HC is common to both heads)

But I am not sure what these are used for.

2) running the tape at a constant 11ips. There is a circuit that uses the tachometer wheel to control the tape speed, but again I am not sure how to program (?) the tape controller IC (U1) to move the tape. (I may need to disable the BOT/EOT sensors, but that should not be a problem).

I am lucky to have a 9915A as my test bed which allows far easier access to the tape drive mechanism than the HP-85 (and does not have these evil ribbon cables).

Unfortunately I do not have a scope, but I can borrow one for a short period if necessary. I have a logic analyzer.

Proposal 2

Another plan is to get the tape moving at 11 ips and then tap the output of the read/write IC (1820-2418) in the HP-85 tape drive to get the bits from the tape.

This allows the use of existing hardware. But still requires programming the tape controller.

I wish I could understand this infernal assembly language of the 85 so that I could look at the internal routines that access the tape for ideas.

Tony suggested to look at the diagrams of the 9815 and 9825 for clues on the tape drive mechanism and the pre-amp. I have looked at them but I do not have access to a 9815 or 9825.

Do note that the speeds of the various tape drive differ:

        HP 9825 22ips
        HP 9835 22ips
        HP 9845 22ips
        HP 85   11ips
        HP 9815 10ips

Any comments, words of advice are always welcome.


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