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USB standard clarification

Posted on 2006-07-20
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Last Modified: 2010-04-30
Experts,

Does the USB standard: 1.0,1.1,2.0, specify JUST the speed of the bus?

Is the Protocol the same for all standards EXCEPT USB Hi Speed? It seems this one is NOT backward compatible to slower standard.

Are there special cables for USB 1.0 and 2.0? Specifically, can I use a 1.0 A&B cable in a 2.0 Enhanced bus architecture? It seems as though there is a  2.0 cable that will work, but has higher grade of shielding?

Can Mini A & B connectors be used in USB 1.0?

Does the Bus powered hub only provide UP TO 100mA per port UP TO 400mA total for the entire device, even if there is only 1 peripheral in the hub?

Does the Self powered hub provide UP TO 500mA PER port as an aggregate of 500mA for ALL ports or PER port regardles of the number of ports on the hub?

In other words do the devices share the total 500mA output UP TO 500mA+/- or individually up to the total ports?

Both support Daisy chain of 127 devices. What happens to the power supplied and the sharing of the bus in regards to bus and self powered when these devices are daisy chained?

Are hubs "PASS through" in USB?

Thanks!
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Question by:link334
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by:stevepo
ID: 17149270
USB 1.0 and 1.1 are 12 Mbps max transfer rate
USB 2.0 is 480 Mbps max transfer rate

USB HiSpeed ports ARE backward compatible with USB 1.1 devices but they run at the USB 1.1 speed

You can use a USB 1.1 cable in a 2.0 port but your 2.0 device will only run at 1.1 speeds.

Thats just a brief overview to answer your question.  You can find the answers to all your questions in this Wiki on USB: http://en.wikipedia.org/wiki/Universal_Serial_Bus

Happy Reading!
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by:link334
ID: 17149486
Stevepo,
I appreciate the response. I have read this and several other (sevvvveral) and am seeking answers to these questions that these articles don't spell out as I have asked them.

If it is due to the fact I personal am unable to interpret "read between the lines" from the content in the article then I shall humbly subside.

Again I thank you, but am looking for specific answers not articles.
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by:bhanukir7
ID: 17150119
hi there

coming to the power issue if ur using a self powered hub that would reduce the drawing of power from the computer which  is good. if u try to connect too many usb devices which draw power from ur computer then ur computer may shutdown or the motherboard may get burnt.

Next is say u have two ports on a computer and u have connected two four port hubs to each port. then u can connect totally 127 and 127 devices.

but that would again degrade the performance of the computer as there is only one IRQ available for the USB so all these 127 plus 127 trying to connect and transfer data would be a herculean task.

so to sum up a single usb port can support upto 127 usb devices. but that would put a lot of burden on the computing power.

revert back with any specific questions if this is not ok

bhanu
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by:link334
ID: 17151586
bhanu,

I thank you for the post and scenario. It has helped somewhat with the daisy chaining concept and device limit.

Questions:
"Next is say u have two ports on a computer and u have connected two four port hubs to each port. then u can connect totally 127 and 127 devices."

The USB standard stated power of +5V would be fed with 500mA supplied to devices-----in your example would that mean each downstream device would draw +5V power from each of the 4 ports (+20V Total or 127*+5V) and supply 500mA to each of th 4 ports or does each of the 4 ports (depending on th draw of power from the device get 500mA total for 4 ports NOT 500mA (4*500mA) to each port?

so to sum up a single usb port can support upto 127 usb devices. but that would put a lot of burden on the computing power."

Does the USB controller (on mobo) support (1) Root Hub?
And is the Root Hub = (1) USB port or many? (E.G. 1 controller on mobo = 1 Root Hub = 1 or 4 ports on mobo)
Is on controller = 1 IRQ regardless the number of ports/devices (127 devices in chain=1 IRQ)?

Sorry if these seem laymen to you experts, but the specs seem vague to me.

I greatly appreciate your guidance.

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Rob_Jeffrey earned 500 total points
ID: 17156217
Let me see if I can mess you up here...

USB is a communication standard - Universal Serial Bus.  
The speed, voltage and number of devices on a bus is defined by the standard.

The voltage is 5 volts - this should be constant.  The amount of current will fluxuate based on the devices on the bus (how many, what they take).  Most motherboards supply the 500mV - some powered (considered Active hubs) hubs can supply 5 Amps (5000mV).  If you are running two devices from the bus of a motherboard without a powered hub (non-powered hubs get power from the bus as well) then you need to watch the power consumption.  The motherboard supplies 500mV - a webcam that takes approx 400mV(depending on the camera - this one has a motor that tracks the user when s/he moves) and a gamepad that needs 200mV.  The two devices will not work from the bus - there is just not enough power.

Each bus can have a maximum if 127 devices on it.  This is a limitation of the communications scheme.  Normal serial ports are a one - to - one relationship.  One port talks to one device.  USB is one port = many devices.  This means we need to address any one device in some way.  USB actually numbers each device when it connects.  If yo plug in a new camera, for example, it gets the next available number, say 1.  Plug something else in and it becomes 2, and so on.  When one device wants to talk to another - it says - I am device X and I want to send this data to Y.  Only Y will retrieve the data from the bus and respond - I am Y and have received the data (un)sucessfully from X.  Consider this - you have 480Mbps of bandwidth - you plug in 10 Web Cams.  Each camera sends 50Mbps of data into the computer - thats 500Mbps - too much for the bus.  All devices will slow down (or possible not even work at all).  It is best to split them up across as many USB busses as possible.

Now for standard archetecture.  Most motherboards supply more than one USB port - however, this does not mean you have more than one USB bus.  My laptop has 3 USB ports - but only one USB Root Hub - meaning I can only have 127 devices - not the expected 381 (3x127=381).
Electrically, a USB port is simply four wires - +5v, Ground, + Data , and - Data.  If you cut the ends off 2 usb devices and twisted the proper wires together (+data from one to +data with the other and the +data from one of the connectors etc) the devices work without problem.  (There are communications issues at high speeds if too much shielding is removed or otherwise excesive 'noise' is introduced to the bus with long wires etc, but it still works).  That is the minimum that USB hubs need to do.  Powered hubs have more electronics involved since they have to electrically shield the +5V and ground from the bus while adding a new power source to the system.

There is a lot more - but I have run out of time.
Hope this helps to give some background.

Rob
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by:bhanukir7
ID: 17156748
hi there

when we say daisy chain format and when we are connecting so many devices we should use self powered hubs. when i said self power usb hubs i mean that if i have connected a four port usb hub to a single usb hub, that usb hub should have its own power source to let the usb devices work efficiently.

if at all u want to connect 127 devices then the connection scheme would be


pc>>>>usb port 1>>>usb 8 port hub>>each port connected with 8 port hub>>>so 8*8=64 so this way u can connect 64 devices.

this is just a illustration. But if u want to connect so many devices u should be using self power usb's which would be taking care of the power requirements

bhanu
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by:link334
ID: 17157617
Rob Jeffrey,

AWESOME!
That is the type of explanation and information I was looking for and brother you brought it!

Still need help on something you mentioned though:

"The voltage is 5 volts - this should be constant.  The amount of current will fluxuate based on the devices on the bus (how many, what they take)."

Do different devices require more Volt and or milliAmps?

Most motherboards supply the 500mV - some powered (considered Active hubs) hubs can supply 5 Amps (5000mV).  If you are running two devices from the bus of a motherboard without a powered hub (non-powered hubs get power from the bus as well) then you need to watch the power consumption.  The motherboard supplies 500mV - a webcam that takes approx 400mV(depending on the camera - this one has a motor that tracks the user when s/he moves) and a gamepad that needs 200mV.  The two devices will not work from the bus - there is just not enough power."

Does the power source (500 or 5000) get divided proportionatily to each port regardless the devices demand OR does the device in one port draw what it power it needs and other devices get whats left over up to 500mA?

This is asked because it seem powered hubs do this power division differently than the bus powered do.

Thanks!
Link


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by:Rob_Jeffrey
ID: 17157797
Sorry - I was using mV (milliVolts) when I ment (and should have used mA milliAmps)

Different devices do require different amounts of power.  This is usually stated in the manual, box or online for the device in question.
There should always be 5V unless something is drawing too much current.  Every device that is powered by the USB bus will run from 5V - that is just the USB standard.  If something needs more voltage it will have to convert it from the 5V bus or use an external power supply.

When there is 500mA of current available on the bus for the devices then that is all there is (okay - tollerances may be off any you may have 523mA or only 498mA - but that is the real world).  If a device states it needs 400mA - like in the camera example - then you can only consider 100mA remaining for use with the gamepad.  Since the gamepad needs 200mA - it won't work.  

Electricity follows the path of least resistance.  This means that the device that uses more power - will get more power.
This is where things get confusing, however.  This doesn't mean that the camera will work while the gamepad is plugged in.
There is a curve of power consumption which gets complicated.  So bear with me while I leave out some real world complications (such as peak power and tolerances).
While the camera needs 400mA - and starts consuming power from the bus - at around 200mA (300mA remaining) the current draw remaining for the camera is now equivilant to the power requirement for the gamepad - so the gamepad and the camera will now both draw current at the same rate.  This means that the current on the bus will run out when the camera reaches 350mA and the gamepad reaches 150mA - both devices are underpowered.

This is extremely simplified - please keep that in mind - and may not be completly correct.

Rob
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by:link334
ID: 17161328
Rob,

That is a "Crystal Clear" explanation to the USB standards I have read and wanted clarification on from my reading.

Fantastic Job!
GRADE A

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by:Rob_Jeffrey
ID: 17161829
Thank you.

I'm glad to have helped.
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