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Old 10-28-2006, 07:57 AM   #16
tw
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Quote:
Originally Posted by zippyt
How do you know how much total load you can put on the USB ports?
USB solved many problems with the RS-232 ports. For example USB devices can be powered from that serial port and all connections / disconnections can be performed 'hot'. All devices today (including the power supply inside all computers) must neither cause damage nor interruption of devices upstream during a short circuit. And USB addresses another problem (irrelevant to this thread) solved by differential mode signaling.

I'm doing this from memory so take them as ballpark numbers. The USB port can provide 100 ma to each port or 1 USB load. All USB devices start in this 'low power' mode. A USB device must than ask for more power while only using less than 100 ma. USB port then enables more power and USB device then shifts to a 'high power' mode. Designs / standards have already determined what is sufficient and what therefore can work. Most USB devices reviewed at IC level have two power levels - 100 ma and 500 ma - even though USB devices can ask for various intermediate power levels.

If a USB device draws too much power, then USB port typically informs (interrupts) computer which then orders power limited. Suspend mode is typically less than 1 ma. To a user, the USB port has failed with a message about a 'surge'.

Above describes a USB device powered via its USB plug. USB can be daisy chained to operate up to 128 devices. Obviously too many devices daisy chained from one USB root port might overwhelm that root port - cause excessive power consumption and shut down that port.

So we have alternatives. Each hub (that daisy chains out to other USB ports) can be self powered. A self powered hub will not draw too much power from the root port. Many self powered hubs use power bricks (wall warts).

A solution for busterb solves that wall wart problem by drawing power, instead, directly from computer's power supply. Obviously this solution also costs less money. It also puts USB ports on computer front panel where connecting and disconnecting is easiest. Another alternative is a PCI card which then puts USB connectors on a more congested IO port in back and requires software configuration (which normally - but not always - would occur automatically). Both solutions solve wall wart congestion.

Meanwhile, USB devices can also provide their own power - such as cameras. Then USB port would not have to provide high power.

USB must be designed so that user can make or break connections 'hot'. 'Hot popping' is involves special design considerations - be it a serial port or a computer PCI card. "Hot popping' permitted by USB. For example, if a device first connects or if a device requests high power mode, then a sudden inrush of current might cause overcurrent. It might even cause a computer crash - if not designed accordingly. USB design makes 'load changing while hot' irrelevant to users.

Some have upgraded their software or connected some devices only to suffer the 'power surge' message and a USB port shutdown. Well, some designs are too subjective - and therefore did not provide sufficient margin of error. Some upgraded software responded too quickly to an excessive 'inrush' current - programmers who don't learn about hardware realities. IOW the user is now at the mercy of an analog world, confused by too many without a full hardware / software grasp, blaming others, and leaving a user with few solutions.

The 'power surge' failure message is not AC electric. It is a USB subsystem doing something often traceable to a problem you can only fix by speculation and shotgunning. USB so simple that when it fails, you have few solutions. But USB is made so simple that such failures should be rare or directly traceable to a human buying only on price.

One related topic not relevant here - USB 1.1 verses USB 2.0. QED - which I also will not define.
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Old 10-28-2006, 04:46 PM   #17
zippyt
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Thanks Tw for yet another informitive post , I always woundered about that ,

Now USB 1 v USB 2 , whats the deal ??
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Old 10-29-2006, 01:02 AM   #18
tw
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Quote:
Originally Posted by zippyt
Now USB 1 v USB 2 , whats the deal ??
USB 1.1 or USB 2.0. Latter is significantly faster which is why Firewire is now so much a niche product promoted by too few vendors (Apple, Sony, etc). Firewire's initial advantage was speed. USB 2.0 eliminated that advantage.

USB 2.0 uses diferent standards from USB 1.1. To get to USB 2.0, first, communication negotiates between device and port using USB 1.1 hardware. This means USB 2.0 ports and devices must support both USB 1.1 and 2.0 hardware - two completely separate functions inside a same chip. Even interface driver and receivers (the semiconductor that connect to USB cable copper wire) for USB 1.1 and 2.0 are different.

To you, any detachable USB cable for low, full, and high speed will be USB 2.0 standard with a type A and a type B connector at each end. Low speed only USB 1.1 cables will not have a standard USB connectors at one end. IOW a low speed USB will connect to a hub (type A) but will not have the USB standard 'B' type connector for device end. B type connector is the more square type connector. A type is the larger more rectangle type connector.

USB extension cords are prohibited. Maximum cable length is determined by mechanical paramters inside each cable. Maximum cable length is not defined by USB standards. Length is defined by the cable manufacturer whose cable must meet USB electrical signal parameters. If he can make a 50 foot cable to meet those parameters, then he can sell it as a USB cable. Don't expect to find 50 foot USB 2.0 cables which are typically 5 meters or less. USB extenders for up to 100 and 500 meters: USB Extenders use ethernet or fiber optics

Whereas 'low speed' in USB 1.1 was up to 100 kilobits per second (analog modem speeds) and 'full speed' USB 1.1 was 12 kilobits per second (writing at diskette speed); 'high speed' USB 2.0 is 400 Mb/sec (faster than ethernet speed).

Whereas classic RS-232 software was trivial - I would write serial port code in assembly. USB 2.0 software is about as complex as MS-D0S due to the so many things that simple serial port does.
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Old 10-29-2006, 03:39 PM   #19
zippyt
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Thanks Tw for yet another informitive post ,

Verry cool !!!
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Old 10-29-2006, 10:41 PM   #20
tw
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Correction is in boldface: Whereas 'low speed' in USB 1.1 was up to 100 kilobits per second (analog modem speeds) and 'full speed' USB 1.1 was 12 Megabits per second (writing at diskette speed);
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