Comment by can16358p
4 years ago
So on the next versions of USB, the cable length will get shorter and shorter until the max gets to 5cm?
While I get the technical reasoning about high frequency/attenuation etc that limits cable length as speeds go higher, there are obviously some practical limits to how short cables can be.
How would that be solved, I don't know.
Keep the same speeds, add more wires.
Printers hate this trick
https://www.unm.edu/~tbeach/terms/images/oldportsBIG.jpg
I'm confused what that section is supposed to represent. E.g. Apple has a 3 meter USB 4 3x2 (40 Gbps) cable but the "cable" value for that section is listed as 0.8m. The only hit I'm getting in the USB 4 spec for "0.8" is on page 59 referring to maximum receiver insertion loss in dB for a gen 3 connection including a 0.8m passive cable but that in itself isn't a hard limitation on cable length.
Not my area of expertise, but maybe some (unrealistic) options include using fiber optics for the data lines, or adding more data lines.
There already exists some fiber-optic USB cables that come in lengths >50m and with support for USB 3.1 so it doesn't seem like a very unrealistic option.
Redmere chips also proved HDMI can go very, very far with a little extra investment. I've run 4K signals hundreds of feet with them. We've seen this problem solved several times, I can't imagine it's physically impossible with USB-C.
That sounds more like fiber optic adapters/converters that fit into usb-ports and talk usb, rather than USB-cables that can be 50+ meters.
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I guess at some point optical will be the only way forward.
Having more data lines in a serial bus is interesting, as the whole reasoning to go from parallel lines (e.g. Centronics, ATA/SCSI or ISA/PCI buses) to serial (SATA/SAS, PCIe, USB) was that coordinating multiple data lines got impossible due to physical limitations where e.g. minimal differences in cable lengths started to matter).
Multiple serial busses, each with its own clocking and buffer, so that the combined data is extracted synchronously at the end. The crosstalk is still a problem but there are ways around that: different twist rates for different pairs for instance.
> I guess at some point optical will be the only way forward.
Maybe. Though Infiniband's currently at 100Gbps per lane on a 1.5 meter passive cable. And active cables can give you a moderate boost while still on copper.
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