TU58 yet one more time

Mattis Lind mattislind at gmail.com
Tue May 31 02:07:24 CDT 2016

tisdag 31 maj 2016 skrev shadoooo <shadoooo at gmail.com>:

> Hello,
> I'm very interested on TU58, as I have a lot of tapes that I need to dump,
> plus I would create some new (console at start) for my vax 730.

Great!  That is exactly what I want to do. But I also like to recover other
tapes from for example HP 9825 / HP 2645. I have studied the QIC-117 spec
some more but concluded that the drive is not dumb enough. The forward
physical and rewind physical only work with full speed and it isn't
specified if any data is output during these commands. However if one
sacrifice the entire drive and just use the mechanics it would be really
dumb so that could be a route. Formatting wouldn't work using this type of
drive since the width of the magnetic head is different.

> What I'm trying to do: use a drive mechanism connected with to modern mixed
> signal microcontroller with USB port.
> The device will be able to control the tape speed and to sample with ADC
> and DAC the analog signals on the read / write heads, transferring the bare
> samples with almost no filtering to a PC.

The bit period is 41.2 us nominally and it is using 1/3 2/3 encoding which
means that there are two flux transitions per bit one at the start and one
at either 1/3 or 2/3 of the bit. So I guess that to decode it one would
like to have at least 2 samples per each third or 6 samples per bit. That
would mean around 160 kHz sampling rate.

For writing I think that it is more on/off switching of a couple of
transistors to have the flux go either way.

> Then all the processing can be done via software.
> In this way, a weak data track can be maybe recovered, and an empty tape
> can be formatted.

One thing to think of is that a TU58 drive does not have optical BOT / EOT
detectors. They make use of special patterns on the tape for this. So to
format a blank tape can be a tricky operation, possibly manually
positioning the tape and then just write the nominal number of blocks and
hope the tape will be long enough.

> It's impossible to use the tape drive with a DEC machine or via serial
> port, but I'm not trying to emulate the standard TU58.
> If somebody is interested, I can share my ideas, I already begun with the
> hardware.
> But I have to fix the capstan too...

Looking forward hearing more about this!

> Andrea


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