Hi,
I want to build my own usb hub. I know I can buy these in the shops
but I want to see if I can make a 24 port usb hub cheaper than I can
buy one.
Can anyone help me in starting this project. All I want is to be point
to a tutorial that will take me through the basic build of a usb
circuit broad. I have a basic understanding of electronics.
Ta Russty
You cascade enough chips, until you get the desired number of ports.
This example chip is a 1:4, and using those, you'd need about 8 chips,
1 at top level, 2 chips at next level, 5 chips at bottom level, for
a total of 25 ports. Chips with larger fanouts (more output ports)
will make this easier. *If* you could find a chip with 1:8, then
two levels would cover it.
http://www.altavista.com/web/results?q=alcor+usb+hub+schematic
http://logout.sh/computers/projects/usbradio/AU9254 R2-020108.pdf
For layout technique, I would treat the USB diff pairs with respect.
A four layer microstrip line PCB would probably do a good job. The
USB spec presumably has some controlled impedance targets for the
differential impedance, and that is what you'd be trying to adhere
to. You might be able to get away with using ribbon cable, to simulate
the same kind of transmission environment. So there might be a
"hobbyist way" to do the project.
Here is another "USB hub controller". This one uses Micrel 2026 to
switch power to the output USB ports. This one also uses a couple
lower voltage regulators, so it looks like the SMSC is more
interested in power management.
http://www.smsc.com/main/tools/usb/usb20h04evb.pdf
http://www.smsc.com/main/datasheets/20h04.pdf
http://www.micrel.com/page.do?page=/product-info/products/mic2026.shtml
Some other notes.
http://www.usb.org/developers/whitepapers/power_delivery_motherboards.pdf
That one makes reference to this. This one describes using a 4 layer
PCB to do a good job of carrying USB2 signals. Something an engineer
might follow (as well as reading the full USB20.pdf spec).
http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf
There is probably a copy of usb20.pdf kicking about, and that is
the thick and meaty USB2 spec from usb.org.
It is not impossible to do this project. You can probably breadboard
the thing. I'm just not sure exactly what technique would make a
good substitute for a 4 layer microstrip PCB. Ribbon cable might
be in the right ballpark for use in carrying the USB signals from
one chip to the next. You'd have to pick the right signal
ground ratio. So that might be the only tricky part. If you
don't get the wiring right between chips, you could end up getting
a lot of CRC errors on USB packets, which will make your new box
"flake city".
If you shop hard, you can probably find some imported hubs for
a low price. Buying USB cables at retail might be the more
expensive part of the project. The trick would be to find
some quality USB cables that aren't too expensive, to build
a cascade of them.
Paul
