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Old 18-02-2020, 06:33 PM
I.C.D (Ian)
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USB cable size

Hi All I read the other day that any USB cable over 2 m should not be any smaller then 25AWG ,as anything smaller will slowdown any information on that cable,is this true
Ian
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Old 18-02-2020, 07:28 PM
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skysurfer
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Depends how much current passes through the cable.
I don't know what an AWG is (is it a length unit depicting the length of a cable? ), but the thicker the wire is, more current can pass through it.
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Old 18-02-2020, 07:36 PM
Startrek (Martin)
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Ian
I’m an imager and observer and have used 5m USB2.0 and USB3.0 cables ( 22/28 AWG conductor sizes ) for various astrophotography hardware and software programs ( cameras, mounts etc.. ) without any issues at all I’ve 3 years
I think a key factor is cable quality ie: conductors with 99% pure copper conductors , quality shielding and or braiding , 90 deg bend tolerance of cable etc....
There are good cables and there are poor quality cables. For me it was a trial and error process to find good quality cable
Hope others can offer some advice too
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Old 18-02-2020, 07:55 PM
Startrek (Martin)
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AWG is an acronym for “ American Wire Gauge”
Most data , communications and instrumentation cables are specified internationally in AWG rather than in sq mm to indicate conductor size
Power cables both extra low voltage and low voltage are generally AS/ NZS specified in conductor size using sq mm
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Old 18-02-2020, 09:23 PM
PaulSthcoast (Paul)
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Hi Ian,

I hope this may help ?

Info via a quick Google search, but I never go over five metres when using a non powered USB cable,
they just fail for me after that length, so its not a matter of choice, its a matter of fact !

I have a mate that uses powered USB cables.....I guess he still hasn't got his heard around Stellamate ?

Maximum Lengths:
According to USB.org, USB operating specifications limit the maximum length of the cable to 5 meters for
full speed devices and 3 meters for any low speed devices. 5 meters is around 16 feet 5 inches,
and 3 meters is around 9 feet 10 inches.

Powered cables:
You can use extension cables and self-powered USB hubs connected together to extend the range of your USB device.
However, it is important to remember when using 2.0 hubs and cables that the distance between each powered hub
can be no more than 5 meters (16 feet and 5 inches).

What is the longest USB extension cable?:
If you are using a regular cable (max length of 5 meters for 2.0 and max length of 3 meters for 3.0/3.1) with an active cable, then the maximum length for USB 2.0 is 25 meters (about 82 feet) and the maximum recommended length for USB 3.0/3.1 is 15 meters (about 49 feet).

Paul.
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Old 18-02-2020, 10:59 PM
gary
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Quote:
Originally Posted by I.C.D View Post
Hi All I read the other day that any USB cable over 2 m should not be any smaller then 25AWG ,as anything smaller will slowdown any information on that cable,is this true
Ian
Hi Ian,

The real problem, as I say to fellow electrical engineering colleagues,
is that when it comes to the speed of light, God is a real underachiever.

Forget cable AWG's and all that gobbledegook for one moment.

We regard USB cables as what we formally term "transmission lines".

As you are aware, digital signals are transmitted as pulse waveforms.

In a vacuum, light takes about 1 nanosecond to propagate one foot.

In a cable, a signal travels a little slower.

Depending upon their surrounding medium (dielectric constant of
insulation and so on), a signal will take about 1.4 nanoseconds to travel
one foot.

When you consider today's high speed circuits, that is pathetically slow.
I mean, what was God thinking when he came up with 3x10e8 m/s
for being the fastest speed that can be? It's a real problem these days.

In particular, the problem for electrical engineers comes about when
we have signals whose rise times (that is the time it takes to transition
from a zero to a one or a one to a zero) becomes similar in magnitude
to the propagation delay time of a cable.

Man wants to change the signal and routinely builds circuits easily
capable of doing so, but God is still taking his time propagating an earlier
pulse down the cable.

It's not just the cables resistance that comes into play, but its
impedance which includes inductive and capacitive effects.

So it's also not the cables current carrying capacity that is the limitation.
It is the fact that it acts as a transmission line. Eventually you can
push the transmission like both mathematically and physically to the point
that the signal at the receiving end is so distorted and has so much
interfered with itself as to become unrecoverable.

That's why USB cables are only so long before needing repeaters.

Best Regards

Gary Kopff
Member Institute of Electrical & Electronics Engineers (IEEE) 42 years
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