B
Bill Bonde { ''Direct And On Point'')
I'm reading the following from usb.org and they seem determined to
prevent anyone from making cables longer than 5 metres. But if you
look at their protestations, isn't it true that the timing spec
issues aren't based on 5 metres but instead on a worst case
scenario with 5 hubs and 5 cables? That would be more like 25
metres.
So this leads to the question, at 5 metres, is the problem with the
USB data lines or with the five volt power lines? If it's the power
lines, could one provide the five volts to the device directly and
create a longer cable for the data? If not, why not?
http://www.usb.org/developers/usbfaq/#sig7
#begin quote
Cables and Long-Haul Solutions
1. Why are there cable length limits, and what are they?
A: The cable length was limited by a cable delay spec of 26ns to
allow for reflections to settle at the transmitter before the next
bit was sent. Since USB uses source termination and voltage-mode
drivers, this has to be the case, otherwise reflections can pile up
and blow the driver. This does not mean the line voltage has fully
settled by the end of the bit; with worst-case undertermination.
However, there's been enough damping by the end of the bit that the
reflection amplitude has been reduced to manageable levels. The low
speed cable length was limited to 18ns to keep transmission line
effects from impacting low speed signals.
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2. I want to build a cable longer than 5 meters, why won't this
work?
A: Even if you violated the spec, it literally wouldn't get you
very far. Assuming worst-case delay times, a full speed device at
the bottom of 5 hubs and cables has a timeout margin of 280ps.
Reducing this margin to 0ps would only give you an extra 5cm, which
is hardly worth the trouble.
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3. What about using USB signal repeaters to make a cable longer
than 5 meters?
A: Don't bother. The best solution is self-powered hub with a
fixed 10m cable that had a one-port bus powered hub in the middle.
The maximum range will still have to deal with the timeout, so any
out of spec tweaking of the terminations between the two hubs and
the timing budget still won't yield more than 5cm of extra
distance. A better solution is described in the following question.
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4. I really need to put a USB device more than 30 meters away
from my PC. What should I do?
A: Build a USB bridge that acts as a USB device on one side and
has a USB host controller at the other end. Use a long-haul
signaling protocol like Ethernet or RS-485 in the middle. Using
cables or short-haul fiber, you can get ranges upwards of a
kilometer, though there's no reason why the long-haul link in the
middle of the bridge couldn't be a pair of radio transceivers or
satellite modems.
Embedded host solutions capable of doing this already exist. Also,
two PCs connected via USB Ethernet adapters are essentially a
slave/slave version of this master/slave bridge.
top top
#end quote
prevent anyone from making cables longer than 5 metres. But if you
look at their protestations, isn't it true that the timing spec
issues aren't based on 5 metres but instead on a worst case
scenario with 5 hubs and 5 cables? That would be more like 25
metres.
So this leads to the question, at 5 metres, is the problem with the
USB data lines or with the five volt power lines? If it's the power
lines, could one provide the five volts to the device directly and
create a longer cable for the data? If not, why not?
http://www.usb.org/developers/usbfaq/#sig7
#begin quote
Cables and Long-Haul Solutions
1. Why are there cable length limits, and what are they?
A: The cable length was limited by a cable delay spec of 26ns to
allow for reflections to settle at the transmitter before the next
bit was sent. Since USB uses source termination and voltage-mode
drivers, this has to be the case, otherwise reflections can pile up
and blow the driver. This does not mean the line voltage has fully
settled by the end of the bit; with worst-case undertermination.
However, there's been enough damping by the end of the bit that the
reflection amplitude has been reduced to manageable levels. The low
speed cable length was limited to 18ns to keep transmission line
effects from impacting low speed signals.
top top
gray line
2. I want to build a cable longer than 5 meters, why won't this
work?
A: Even if you violated the spec, it literally wouldn't get you
very far. Assuming worst-case delay times, a full speed device at
the bottom of 5 hubs and cables has a timeout margin of 280ps.
Reducing this margin to 0ps would only give you an extra 5cm, which
is hardly worth the trouble.
top top
gray line
3. What about using USB signal repeaters to make a cable longer
than 5 meters?
A: Don't bother. The best solution is self-powered hub with a
fixed 10m cable that had a one-port bus powered hub in the middle.
The maximum range will still have to deal with the timeout, so any
out of spec tweaking of the terminations between the two hubs and
the timing budget still won't yield more than 5cm of extra
distance. A better solution is described in the following question.
top top
gray line
4. I really need to put a USB device more than 30 meters away
from my PC. What should I do?
A: Build a USB bridge that acts as a USB device on one side and
has a USB host controller at the other end. Use a long-haul
signaling protocol like Ethernet or RS-485 in the middle. Using
cables or short-haul fiber, you can get ranges upwards of a
kilometer, though there's no reason why the long-haul link in the
middle of the bridge couldn't be a pair of radio transceivers or
satellite modems.
Embedded host solutions capable of doing this already exist. Also,
two PCs connected via USB Ethernet adapters are essentially a
slave/slave version of this master/slave bridge.
top top
#end quote
