Is USB balanced or not?
It uses differential signaling so you would think it would also be balanced, but from what I'm reading and seeing in wiring diagrams, it's absolutely not.
The standard looks thick for such a simple question, BTW.
I'm not sure what you mean. What is the difference between differential and balanced? What do you see in the standard that indicates it is not balanced? Do you mean the signals are purely differential and not equal magnitude with opposite polarity wrt ground? The reference point is not ground, but does that mean the signals are not balanced?
-- Rick
Den torsdag den 29. oktober 2015 kl. 19.45.43 UTC+1 skrev Aleksandar Kuktin:
USB data is differential, but end of each packet is a single ended zero
-Lasse
OK, thanks.
The difference is in the common mode voltage. If the line is balanced, activity on the line will not produce any common mode voltage and vice versa. Appearance of common-mode voltage is ofcourse a bad idea because then your cable both emits EMI and is susceptible to it.
Just to give a complete answer in any case: common mode voltage is one present on both lines simultaneously. You can measure it by making a symmetrical voltage divider and measuring the voltage between the dividers midpoint and a reference point, say ground of the transmitter or receiver.
10BASE-T ethernet is an example of an interface which operates at a similar bitrate to full-speed USB (10Mbs versus 12Mbs for USB) and is both differential and balanced.
I haven't actually read the standard, I've been reading on the net about several implementations of USB and looking at their wiring diagrams.
From what I see, none of these are balanced, which I thought was strange, so I came and asked.
For what it's worth, these apply to USB 1.0/1.1/2.0, but not to USB SuperSpeed (3.0/3.1). There the USB 2.0 pins remain unchanged (unbalanced), but the 5Gbps/10Gbps channels are differential and balanced (and the construction puts in a lot of effort to keep it that way).
The USB 1.x/2.0 PHY is... unusual, from a PHY perspective.
Theo
You're being very generous. :)
USB 1.x/2.x is/was utter crap. It had Microsoft and Intel's incompetent fingerprints all over it.
From the PHY design all the way up to the stupid master/slave polling scheme. Firewire was a far, far better design for things like audio, video, networking, and mass storage. Unfortunately it didn't really work well for keyboards and mice. So as a result, we spent decades using trying to use a half-assed low-speed keyboard/mouse protocol to do networking and high speed bulk data transfers.
But I supposed it went well with the half-assed OS and processor architecture most people used it on. But don't get me started on that...
-- Grant
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