HP 7970E - interest to split?

Jon Elson elson at pico-systems.com
Sat Sep 30 12:04:21 CDT 2017

On 09/30/2017 06:58 AM, Tony Duell via cctalk wrote:
> One of my tape drives has a set of equal-value resistors connected to
> each of the read channels, the others ends of the resistors being brought
> out to a testpoint. You connect a 'scope there and read a skewmaster
> tape. The idea is that if the read transitions all occur at the same time
> (no skew) you get a pefect step on the 'scope, if some are later than
> others you get a staircase. You adjust the heads for the smallest
> width of the stairs.
Yes, this is the classic NRZI skew adjust testpoint.  The 
resistors sum up the read data latches, that are set by 
seeing a transition of either polarity on a data channel, 
and reset by a one-shot that is set to about half a 
character time.
You run a skew tape or a tape written with all ones.  If you 
see a square wave, all is good.  If you see a slope on the 
wave, adjust the skew adjust screws on the head mount to 
make it as straight as possible.


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