USB power extensions

[moboking]

I know that the length of the USB cable to carry data reliably is at about 20 feet maximum. This means that a USB printer should not be more than 20 feet away from the PC. However, if I connect the printer to a USB power hub and then connect the hub to the PC, how far can I extend the distance between the printer to the computer? Another 20 feet and make it a total of 40 feet? If this is the case, is there another device, a smart USB hub by any chance that can even extend the distance? What if there are additional USB devices plugged into this USB hub? Would the 40 feet maximum (if that is possible) be lessen?

 

[mpjesse]

I think the only reason USB devices shouldn't be on a cable more than xxx feet is signal degregation. (i.e., at 25 feet speed is 400mbps compared to 480mbps) USB devices have parity checks so corrupt data is unlikely.

Unless speed is extremely important, I wouldn't worry about extending usb cables.

But if you are still concerned, there are USB cable extensions w/ built in repeaters to minimize signal loss. I would suggest getting one of those instead of a hub.

-mpjesse

 

[Crashman]

Good job Jess, I was just going to recommend a repeater...

 

[moboking]

What is the difference between a usb hub and a repeater?

Weird though that the vendor says that I can daisy chain these repeaters all the way to 80 feet and yet none of them requires an external power connector. How can the signal be extented so far?

 

[Crashman]

A reapeater is a signal amplifier. It takes the incomming signal, amplifies it, and sends the amplified signal down the next cable. I guess it would be possible to power a repeater off the 5v line in the USB data cable, but that would limit your length somewhat. Hence the dealer said 80 feet.

If the repeaters had their own power you should be able to extend the signal for miles!

 

[moboking]

See, this is what I do not understand. An amplifier without a "kick" from its own independent power source seems to me like getting out of something from nothing. (Free energy?). An example is a speaker is an amplifier of sound and they all require their own AC adapter or in some cases, battery.

 

[Crashman]

You're missing a point: The USB cable has something like 4 data wires, a 5v power wire, and a ground. You can power a lot of things off the 5v power wire, so long as the things you're powering don't use much energy.

Over longer distances the 5v wire looses voltage due to resistance, hence I said it was understandable why you're dealer would impose such a limit as 80 feet.

edit: the power might go over the data wires, but you can still power things from it. I haven't looked at any USB pin diagrams for a few years.

 

[moboking]

I am aware that the USB cable also provides power to devices that are plug into the port. My point is how is a "repeater" different from an ordinary "extender"? From what it looks, the plugs for both the repeater and the traditional extender are the same. The only difference is the repeater has a bulky encasing at the contact point where the female end is. So my confusion is what does the circuitry in this bulky encasing do to repeat the USB signal that a plain extension cable does not do? Since the reapeater does not have its own power source to give it a kick, it has to somehow uses the pre-existing power from the cable itself. Therefore, a repeater must have some kind of intelligence at the bulky encasing end to refocus and "amplify" the signal since brute force is not an option, right?

 

[recreational_user]

Actually, USB uses four wires: two for powering devices (+5v and ground) and two for data (Data+ and Data-, it is a serial bus after all). A normal extension cable just makes all four wires longer, increasing the resistance and chance for electromagnetic interference. A repeater actually reads the data signal and rebroadcasts it. So in a sense, your computer is now seeing the repeater as the device, rather than the actual device itself, and the device is seeing the repeater as the computer!

"Since the reapeater does not have its own power source to give it a kick, it has to somehow uses the pre-existing power from the cable itself. Therefore, a repeater must have some kind of intelligence at the bulky encasing end to refocus and "amplify" the signal since brute force is not an option, right?"

The repeater is powered off of the same 5v line that your device is powered from, so it does have a power source. Rather than the analog amplification you are thinking of, the digital signal is just read and then retransmitted (or repeated) from scratch down the other end.

As Crashman said, there is a limit to how many bus-powered repeaters (those without an external power supply) can be strung along, as each repeater (along with the added wire resistance) eats some of the finite (500mA I believe) current avaialble.

Hope that helps.

 

[moboking]

So if I use an independently self-powered repeater, I can theoretically stretch the line beyound 80 feet limit? This means I can connect a USB printer hundreds of feet away from the computer? If this is true, then why do we need devices like these so-called USB servers like the one offered by Keyspan?

http://www.keyspan.com/products/usb/server/)

The USB server basically uses Ethernet to carry the signals from USB devices. With self-powered USB reapeaters, there is no need to use a hybrid technology like a USB server, right? Just plug all far away devices to a USB hub, connect the hub to a powered repeater, and run the cable for long distances to a PC.

 

[Crashman]

With a USB server you can have an entire office network connected to a group of USB devices over the network cable that they already have in place. Heck, you could use an old crappy PC as a USB server too.

 

[moboking]

My sentiment exactly. Why need a USB server?

 

[recreational_user]

So if I use an independently self-powered repeater, I can theoretically stretch the line beyound 80 feet limit? This means I can connect a USB printer hundreds of feet away from the computer? If this is true, then why do we need devices like these so-called USB servers like the one offered by Keyspan?

http://www.keyspan.com/products/usb/server/)

The USB server basically uses Ethernet to carry the signals from USB devices. With self-powered USB reapeaters, there is no need to use a hybrid technology like a USB server, right? Just plug all far away devices to a USB hub, connect the hub to a powered repeater, and run the cable for long distances to a PC.

Well, yes and no. You would have to use a series of repeaters, as each bus-powered one will only get you an extra (approx) 20'. Also, every 80' you would probably have to use another powered repeater, since you would be stretching the USB current limit at that point. But in theory, yes, you could do that.
The added bonus to the USB server (besides being able to run on ethernet, which gives you 330' of range) is that you can share all of the devices on the hub with multiple computers, instead of just the one. There are USB switching devices that can share a USB device between multiple computers, but by the time you buy one of those and all of the repeaters, the USB server device would probably be cheaper. That may be a different issue from what you're asking though.

 

[Guest]

If your printer is self powered there probably wont be any problem with pushing the limits as it will just be a data integrity thing and error correction can get the job done. BUT if the printer is powered off the USB bus then it may not work. There is a 4 V minimum requirement on nominally 5V USB power. Below 4V a USB device doesn't have to keep working and still be compliant. If the cable is too long for a high power bus powered device then the cable drop can cause it to fail.

Does your printewr have a power plug that goes to the wall? If yes then you can probably push the line length with success. If not you will probably just have to experiment to see what works.