[GTALUG] War Story: Asus UX305ca SSD failures

[Russell Reiter]

On Sun, Aug 4, 2019, 9:21 AM James Knott via talk <talk at gtalug.org> wrote:

> On 2019-08-04 08:09 AM, Russell Reiter wrote:
> > Also interesting is that NRZI  seems to have two definitions. Non
> > Return Zero Inverted or NRZ-IBM.
> >
>
> NRZI was created by IBM, specifically for use with tape drives.  They
> were one of the earliest, if not earliest to use mag tape.  I read the
> technical reason for "inverted" many years ago, but I have forgotten the
> details.  Often that sort of thing is done to obtain best performance
> from something.  One such example was the use of odd parity.  From a
> strictly error detection point of view odd or even will work, but with
> odd, there will always be one "1" bit for clocking, as I mentioned.
>
> From looking at the manual, inverted might be a reference to their NOR &
> XOR logic gates.


Check out the wiring patches on the unit in the manual you can see what a
> cluster fork that could turn out to be if you had to troubleshoot it,
> especially where line voltage is used for sync.
>

>From your Wikipedia link it is indicated that NRZI was designed to work
> with or without a clock sync. A term I never heard before, off keying,
> refers to using the actual line polarity to determine if the logical state
> is 0 or 1; that is where line clock tic is not used.
>

Here's the second paragraph from your link. There are secondary data sync
methods when there is no specific timing signal multiplexed into the
stream.

"For a given data signaling rate
<https://en.m.wikipedia.org/wiki/Data_signaling_rate>, i.e., bit rate
<https://en.m.wikipedia.org/wiki/Bit_rate>, the NRZ code requires only half
the baseband bandwidth
<https://en.m.wikipedia.org/wiki/Bandwidth_%28signal_processing%29> required
by the Manchester code <https://en.m.wikipedia.org/wiki/Manchester_code> (the
passband bandwidth is the same). When used to represent data in an asynchronous
communication <https://en.m.wikipedia.org/wiki/Asynchronous_communication>
scheme,
the absence of a neutral state requires other mechanisms for bit
synchronization when a separate clock signal is not available."

It goes on to say that NRZ draws half the bandwidth of RZ encoding. I guess
that's why there are numerous validation methods built into NRZ. Thats an
attractive feature for in house IT, at a time when they have to cobble
their own systems together with parts from different manufacturers.

>
> Some nice pictures of an IBM unit in this link to a manual, for any
> > other creative anachronists.
> >
> > http://ibm-14In .info/223-6988-729-MagTapeCE-InstRef-62-r.pdf
> > <http://ibm-1401.info/223-6988-729-MagTapeCE-InstRef-62-r.pdf>
> >
>
> I used to work on drives that looked similar.  However, they were made
> by a company called Potter, but had the Collins branding on them.


> How often would you do routine servicing, as opposed to repairs? They look
> like huge dust magnets to me and I can't see dust and magnetic tape playing
> well together.
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