On the Emulation of TU58s

Johnny Billquist bqt at update.uu.se
Tue Jun 16 13:30:48 CDT 2015

On 2015-06-16 20:23, Mark J. Blair wrote:
>> On Jun 16, 2015, at 11:22 , Johnny Billquist <bqt at update.uu.se> wrote:
>>> Pretty much the standard CDC vacuum-column design since the 1960s.  The
>>> TU80 is, after all, a CDC product.  Two counter-rotating capstans--tape
>>> movement is determined by applying positive or negative pressure to
>>> ether or both (i.e. positive pressure on both results in the tape
>>> "floating"
>> Uh... The TU80/TU81 do not have vacuum columns...
> Or rotating capstans...

Bu the way. For the vacuum column drives that I know 
(TU16/TU45/TU77/TU78), tape movement is not determined by vacuum as such.

Each column have about 8 pressure switches. When 4 have low pressure, 
and 4 have atmospheric pressure, the drive is happey, and the reels are 
resting. When the point of underpressure/pressure moves down, the reel 
on that side starts moving to pick up more tape, thus moving the point 
back towards neutral. When the point moves up, the reel starts moving to 
give out more tape, to move the point back down again. The more switches 
triggered in either direction, the faster the reel will move.

The movement of the tape across the head is controlled by a third motor, 
which have a smaller wheel on it. The tape lies against this reel, but 
it's all done by friction. This wheel pretty much runs the tape across 
the head at constant speed, and this movement of the tape will also 
cause the points in the two columns to move, which in turn will trigger 
action as described in the previous section.

Not sure where the counter-rotating capstans would fit into this either...
And there is only negative pressure, and it is applied to the tape at 
all time.


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