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Re: USB cabling constraints

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On 2/8/2017 4:49 PM, Tim Williams wrote:
> "Don Y" <blockedofcourse@foo.invalid> wrote in message
> news:o7e3iu$3k3$1@dont-email.me...
>> But, he can produce something that is total shit.  That falls apart during
>> "normal" (what's that?) use.  Or, whose performance degrades quickly
>> (shields opening up from flexure).  Or, outright failures without being
>> unduly stressed (connections opening/wires fraying from handling).
>
> Standards for all these things exist, along with the means to perform the
> tests.  Example: flexure test, consisting of wobbling the cable back and forth
> (do a test at the connector end, too, to see the efficacy of the strain relief
> and braid-to-housing join), for a suitable bend radius and number of cycles.
> Qualification consists of DC checks (resistance per conductor, no shorts
> between wires), and perhaps a nice RF spot-check too (e.g., impedance within
> tolerance, shield attenuation >60dB, etc.).
>
> I don't know offhand what is recommended, but I expect the USB standard has
> some preferences; if not (or, if not sufficient for your purposes), you can
> surely find better specs, all the way up to mil spec (which is free, also!), if
> you like. :)

The standard spends a lot of time on electrical and mechanical details
(dimensionality) and a *wee* bit on suggested materials, etc.  Of course,
invariably qualified with "or better".

It addresses number of mating cycles (at a maximum frequency of occurrence),
connector retension force, etc.

But, it's also abundantly clear that "practice" deviates considerably from
"compliance".  E.g., many of the things EXPRESSLY FORBIDDEN by the standard
are easy to buy off-the-shelf.  Ooops!

Note the number of drek products out there that give the impression of being
"compatible" yet clearly aren't fulfilling even the most basic of the criteria
laid out (wire gauge, etc.)

And (more below), have you ever seen a cable that CLAIMED to embody the
standard -- AND NOTHING MORE?  (i.e., how else would you know the standard
was "sufficient" for your particular needs?)

> Keep in mind that stacking specs will likely give you a higher cost.  You can
> just as well pass that off as in-house quality checks (even better! --  but
> more expensive over time), or as qualification testing from a few different
> suppliers and sticking with one (presumably doing spot checks every so often to
> determine if they've cheaped their product on you, and it's time to search again).

A reply to the post in another forum suggested approaching cable manufacturers
for "samples" -- at a variety of quality levels/price points.  Then, testing
these for suitability.  Unfortunately, it would tend to require larger
populations than just "samples" to account for expected variation between
units.

So, I'll tackle it in two steps:
- samples to get a feel for the range of offerings (between vendors
   and within a vendor's capabilities matrix)
- sample *lots* for select vendors that can be more explicitly measured
- codify requirements based on an assessment of the characteristics of
   the "most appropriate" sample lots purchased
- develop a procedure to test/inspect production units to verify that
   compliance

<frown>  I guess its sort of like trying to evaluate the quality of a
*hammer*...

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Thread

USB cabling constraints Don Y <blockedofcourse@foo.invalid> - 2017-02-07 08:43 -0700
  Re: USB cabling constraints amdx <nojunk@knology.net> - 2017-02-07 10:28 -0600
    Re: USB cabling constraints jrwalliker@gmail.com - 2017-02-07 08:54 -0800
    Re: USB cabling constraints rickman <gnuarm@gmail.com> - 2017-02-07 20:37 -0500
    Re: USB cabling constraints Don Y <blockedofcourse@foo.invalid> - 2017-02-07 20:23 -0700
    Re: USB cabling constraints Robert Baer <robertbaer@localnet.com> - 2017-02-08 21:29 -0800
      Re: USB cabling constraints Kamen Lilov <kamenl@gmail.com> - 2017-02-09 14:01 -0800
  Re: USB cabling constraints Jeff Liebermann <jeffl@cruzio.com> - 2017-02-07 10:39 -0800
  Re: USB cabling constraints "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-07 19:06 -0600
    Re: USB cabling constraints Don Y <blockedofcourse@foo.invalid> - 2017-02-07 20:35 -0700
      Re: USB cabling constraints "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-08 17:49 -0600
        Re: USB cabling constraints Don Y <blockedofcourse@foo.invalid> - 2017-02-08 20:33 -0700
          Re: USB cabling constraints "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-09 16:43 -0600
            Re: USB cabling constraints Don Y <blockedofcourse@foo.invalid> - 2017-02-09 16:18 -0700

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