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)



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 Wulfraed                 Dennis Lee Bieber         AF6VN
 wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/

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