Diy Y usb cable

[R1200CL]

[mansr]

24 minutes ago, R1200CL said:

I like it 😀

 

So how long does it have to be in order to comply with the USB specifications ? .....

There is no minimum length.

[sbronf]

33 minutes ago, R1200CL said:

 

I like it 😀

 

So how long does it have to be in order to comply with the USB specifications ? .....

 

I think it has everything a USB cable should have: right twisting, right awg, right pte insulation.

no lenght requirements on usb.org specs

[R1200CL]

7 minutes ago, mansr said:

There is no minimum length.

 

So where is the 90 ohm located ? In the cable or the plugs ?

[sbronf]

In the geometry of conductors( twisted pairs (tot twists per ft), in the dielectric carachteristichs and dielectric diameter

[mansr]

1 minute ago, R1200CL said:

So where is the 90 ohm located ? In the cable or the plugs ?

90 Ω is the characteristic impedance, not DC resistance.

[sbronf]

6 minutes ago, R1200CL said:

 

So where is the 90 ohm located ? In the cable or the plugs ?

It's the square root of the inductance divided by capacitance

[sbronf]

Edit

[Speedskater]

4 hours ago, sandyk said:

 This isn't the appropriate Forum to discus this kind of subject.

I'm not the one that posted that small sub-set of 'ground' symbols.

 

4 hours ago, sandyk said:

Neither does it have any relevance to the question originally asked by the OP.

No it doesn't, but then I'm only correcting other posts.

[Speedskater]

The 90 Ohms is the Radio Frequency Characteristic Impedance.  It only matters at radio frequencies (like well above 100kHz) and at digital frequencies. It's can't be measured with a simple Ohm meter.

The longer the cable the more it matters. In a 1 meter cable not so much, but in a 10 meter cable it matters.

[R1200CL]

Now it’s getting interesting. 

 

So you may use one twisted pair in an ethernet cable as one conductor. 

So maybe @Cornan‘s home made USB isn’t so bad after all. 😀

 

 

 

 

[mansr]

28 minutes ago, R1200CL said:

Several entries in that table are wrong. HDMI, DisplayPort, DVI, and PCIe should all be 100 Ω. The figures there are taken from an NXP application note referencing an Intel design guide which recommends using slightly lower values for PCB traces in specific applications. Don't trust Wikipedia.

[R1200CL]

@mansr

The use of a twisted ethernet cable as USB was the interesting part here.  So I guess the value for ethernet was correct. 

 

And next question is, why do one have to comply with the specifications?

And more important what happens if you don’t ?

 

Bits are bits 😂

[mansr]

36 minutes ago, R1200CL said:

@mansr

The use of a twisted ethernet cable as USB was the interesting part here.  So I guess the value for ethernet was correct. 

Yes, Ethernet is 100 Ω. That's (barely) within the acceptable range for USB 2.0.

 

Quote

And next question is, why do one have to comply with the specifications?

And more important what happens if you don’t ?

Then you get signal reflections which, if bad enough, cause transmission errors.

 

Quote

Bits are bits 😂

Only if they are received correctly. Impedance matching is one part of ensuring that they are.

[marce]

12 hours ago, R1200CL said:

@mansr

The use of a twisted ethernet cable as USB was the interesting part here.  So I guess the value for ethernet was correct. 

 

And next question is, why do one have to comply with the specifications?

And more important what happens if you don’t ?

 

Bits are bits 😂

Where to start, the whole field of signal integrity/EMC studies is huge...

Here is a nice simple article:

https://www.rogerscorp.com/documents/2920/acm/articles/When-is-Controlled-Impedance-Important.pdf

References to PCB's are relevant for the cable, a cable is just an extension of the PCB tracks to get the signal to some other point.

If you want to go further there are numerous sites with both beginner and advanced information:

https://www.rogerscorp.com/documents/2920/acm/articles/When-is-Controlled-Impedance-Important.pdf

https://www.bethesignal.com/bogatin/

https://ralphmorrison.com/events.php

 

Numerous app. notes, data sheets, guides, references, studies, signal speeds are getting faster, USB 3 requires far tighter design than USB 2, the faster the signal (this is the signals rise time) the more critical the impedance control.

 

[mansr]

2 hours ago, marce said:

the faster the signal (this is the signals rise time) the more critical the impedance control.

And for this reason, picosecond rise times on signals with fairly low edge rate (like S/PDIF) isn't necessarily a good idea.

[R1200CL]

33 minutes ago, mansr said:

And for this reason, picosecond rise times on signals with fairly low edge rate (like S/PDIF) isn't necessarily a good idea.

 

Did you just made an argument why USB is likly to be superior to S/PDIF ?😀

[mansr]

35 minutes ago, R1200CL said:

Did you just made an argument why USB is likly to be superior to S/PDIF ?😀

No.

[marce]

1 hour ago, mansr said:

And for this reason, picosecond rise times on signals with fairly low edge rate (like S/PDIF) isn't necessarily a good idea.

Super fast rise times are never a good idea, but again in this hobby there is a belief that the squarer the edges the better the signal... The slower the rising and falling edges the better, again picking the correct edge speed is another aspect of signal integrity and if the drive capability of the gate isn't changeable you can always put a series resistor as close to the output pin as possible...

 

17 hours ago, mansr said:

Several entries in that table are wrong. HDMI, DisplayPort, DVI, and PCIe should all be 100 Ω. The figures there are taken from an NXP application note referencing an Intel design guide which recommends using slightly lower values for PCB traces in specific applications. Don't trust Wikipedia.

PCIe seems to depend on the number of layers in the PCB, with 100R being recomended for low layer counts (4-6) and 85R for higher layer counts (8-10). Even though the drivers are for 100R it seems you get better signal integrity on high layer count (or longer run backplanes) using 85R diff impedance.