Text 48053, 56 rader
Skriven 2008-08-14 09:25:12 av Jeff Guerdat (1:124/311)
     Kommentar till en text av Björn Forsström
Ärende: Re: USB
===============
Björn Forsström wrote:
>>> Is there a current limit on the USB ports?
> 
>  JG> There's no limit that I'm aware of but that may not count for much.
> 
> There is. I Googled USB and it says 500 mA total.

I took that differently.  Sorry - yes, I knew it was 500mA.

>  JG> 1) Are they daisy-chained or plugged into separate ports?
> 
> Separate ports and I don't think USB can be daisy-chained at all.

Depends on the device.  There are some that have pass-through ports 
built-in.

>  JG> 2) Are the ports powered?
> 
> USB ports are powered with 5 V.

Not all - that's why there's powered vs. unpowered hubs.  If there's no 
power supply for an external hub, it's unpowered and all you get is the 
500mA from the motherboard port to be split between devices attached to 
the hub.  Rather problematic.

>  JG> 3) Are any of the devices plugged into an external hub?
> 
> Nope, and that's just what I have to buy.

Not necessarily - some devices simply don't play nice when plugged into 
a hub.  Read the various device docs first - they may specifically say 
to not use a hub.  My film scanner was a definite - absolutely had to be 
plugged into a motherboard port.  Don't know about my all-in-one printer.

>  JG> I don't know that all USB devices play nice with each other, especially
if
>  JG> not plugged into a motherboard or expansion card port.  Hubs seem to be
>  JG> problematic, especially unpowered ones.  Where at all possible, I try to
>  JG> plug each device into it's own motherboard port.  I would expect that an
>  JG> expansion card such as those Jean uses would work the same as
motherboard
>  JG> ports...
> 
> An optical USB mouse take 100 mA and that's 20% of 500 mA so a powered hub
> is what I need to use all devices at the same time.

You may be better off getting a PCI card for the computer.  More 
cumbersome but also less problematic.

-- 
Jeff Guerdat

Why do they lock gas station bathrooms? Are they afraid someone will
clean them? - George Carlin
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