Dennis Lee Bieber  wrote:
> On Tue, 23 Jun 2020 11:19:37 +0100 (BST), mjb@signal11.invalid (Mike)
> declaimed the following:
>
>>
>> Note, this can't be voltage drop related, as the Topfield is
>> not powered by the PC, data link only!
>
>  You may still have a voltage drop in the 0/1 levels of the data.
> Assuming CMOS 30/70% points, on a 5V system, 0-bits are 1.5V and under,
> 1-bits are 3.5V and above. Voltages in between are indeterminate --
> circuits likely using Schmitt triggers to prevent signal fluctuations when
> in that region.
>
> NOTE: per a Google, USB 1.1 used 0-3.3V, USB 2 uses 0 to 0.4V (I think the
> article implies these are minimum signal change); it also indicates that
> USB uses differential signals, so should be more immune to noise as rather
> than flipping one line between low and high, two lines flip into opposite
> states.
>
https://www.analog.com/en/analog-dialogue/articles/switching-in-usb-consumer-ap
plications.html#
>
>  In a long enough cable, voltage drop could result in a receiver not
> detecting transitions to 1-bits reliably (unless you have some odd coupling
> of a DC signal, the 0-bits are likely reliable)
>
>
>

Voltage drop on data lines due to cable resistance is extremely improbable,
since the current is relatively low.  Put another way, the impedance of the
driving and receiving circuits is high compared to cable resistance.

More likely, capacitance between lines of a longer cable slows signal
transitions, eroding signal margins.

An oscilloscope would reveal the truth.

--
-michael - NadaNet 3.1 and AppleCrate II:  http://michaeljmahon.com

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