0
\$\begingroup\$

I am aware that USB uses a positive, negative and D+/- wires in order to transmit data. The twisted pair are used to negate the effects of interference and electronic noise.

I also understand that a cable needs to be thick enough to carry the necessary current, and connections need to be made properly so that high frequency signals are not reflected etc. Based on my limited knowledge I believe this is due to AC impedance being proportional to frequency.

However, presuming a high enough quality cable, and a fast enough chipset to handle the data, is there any reason why we cannot connect all common devices over such a 4-cable system? (Ignoring the user experience)? Could PCI-E and all other digital serial interfaces be replaced by a fast enough USB like system? If not, why?

\$\endgroup\$
  • \$\begingroup\$ PCIe is a serial bus \$\endgroup\$ – jsotola Jun 7 '19 at 2:26
  • 2
    \$\begingroup\$ You could take a 4-wire cable and use both pairs for data, but then it wouldn't be called USB! I can't at the moment find who said 'What the world wanted was powered Ethernet, what the world got was USB'. Could we make one standard interface that could replace USB, HMDI, Thunderbolt, SATA, Firewire, DVI, PCIe? Sure! \$\endgroup\$ – Neil_UK Jun 7 '19 at 5:24
  • 1
    \$\begingroup\$ uh, PCI-e is essentially a four wire bus. \$\endgroup\$ – Marcus Müller Jun 7 '19 at 8:32
  • \$\begingroup\$ @jsotola - So is USB. \$\endgroup\$ – WhatRoughBeast Jun 7 '19 at 12:05
1
\$\begingroup\$

All data could be sent over 1 wire if you really wanted.

The problem you encounter as you try to send data faster and faster is loss of data integrity. The solution comes as a mix of circuitry at each end and more specialized cables.

You then get faced with a second problem. Specialized cables cost a lot to manufacture. It's going to be pretty hard to convince anyone to use your awesome data transmission method if they have to pay $100/meter for the cable, right? There is also the cost of terminating the cable and how difficult it is to install.

You now have several different groups of people come up with several different solutions to the same problem. All of which think their solution is the better one (or more likely the people who invested in finding the solution want to recoup their investment and don't care which is better.) Thus begins your war of competing solutions which all do the same thing.

And contrary to what you may have been told, the product with the best marketing, lawyers and lobbyists wins. The best product quite often loses.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ over two wires, or one "tube", honestly, because you can't do high speed data over a single conductor; you need to transport the information as EM field/wave between a pair of conductors (such as twisted pair in USB2, Ethernet, DSL) or in a waveguide (most typically, coax cable, but could also simply be a hollow waveguide, but that's balance-wise, conductance-wise and mechanically more challenging. If you attach a high-bandwidth signal source to a single wire, you're not building a wired transceiver; you're building a radio transmitter. \$\endgroup\$ – Marcus Müller Jun 7 '19 at 8:31
  • \$\begingroup\$ @MarcusMüller Unless it is optical \$\endgroup\$ – Dirk Bruere Jun 7 '19 at 9:38
  • \$\begingroup\$ @DirkBruere you mean the EM waveguide commonly known as "fibre"? (one one hand, it's actually guiding EM waves, on the other hand, it's typically not really mode-restricted, so I'm not sure myself whether I could call a single mode fibre a waveguide. I'll ask someone who'll definitely know.) \$\endgroup\$ – Marcus Müller Jun 7 '19 at 11:00
  • 1
    \$\begingroup\$ @Deep it's not quite that simple, adding more wires eventually runs into the same problems. But it's generally a lot easier/cheaper to take your two wire system, add another two wires to it and have your marketing department claim it's 'up to' twice as fast*. Than to actually make your thing twice as fast. It's basically all free market capitalism's fault. \$\endgroup\$ – hekete Jun 7 '19 at 19:05
  • 1
    \$\begingroup\$ Pretty much, or I guess one company could get a total monopoly. Climate change will probably get us before either of those happen though. \$\endgroup\$ – hekete Jun 9 '19 at 5:37
9
\$\begingroup\$

Serial buses are widely used for high speed transfer.
This diagram shows the available data transfer / clocking rate of a number of serial and parallel buses. In addition, it is common to implement multiple "lanes" each using serial transfer.

Serial buses are able, in practice, to implement higher transfer rates "per lane" and even parallel buses with very wide buses (eg PCI 64 bit) are still unable to match the thoughput of the fastest sinle lane serial buses (eg USB 3.1).

enter image description here

The above diagram is from Multi-gigabit serial protocols demystified which addresses your question far better than can be done here.

They say:

... There is a physical limitation on the clock rates of parallel buses at around 1 to 2 GHz. This is because of skew introduced by individual clock and data lines that cause bit errors at faster rates.

High-speed serial buses send encoded data that contains both data and clocking information in a single differential signal, allowing engineers to avoid the speed limitations of parallel buses.

Today, it is common to see high-speed serial links with data lanes running at 10 Gb/sec. Additionally, multiple lanes of serial links can be coherently bonded together to form communication links with higher data throughputs. ..."

Also see:

Web search super high speed serial bus - much good material

|improve this answer|||||
\$\endgroup\$
3
\$\begingroup\$

You could take a 4-wire cable and use both pairs for data, but then it wouldn't be called USB!

I can't at the moment find who said 'What the world wanted was powered Ethernet, what the world got was USB'.

Could we make one standard interface that could replace USB, HMDI, Thunderbolt, SATA, Firewire, DVI, PCIe? Sure thing (obligatory XKCD)

|improve this answer|||||
\$\endgroup\$
3
\$\begingroup\$

Sure, that's exactly what USB 3.0 SuperSpeed and related technologies such as Thunderbolt are all about. It isn't just 4 wires any more, but it is a very high speed serial interface.

Right now, my laptop is driving a 4K display, 1G Ethernet, and a few other peripherals through a single USB-C connector.

|improve this answer|||||
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.