[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

RE: USB 2.0



I also think that there is some confusion between MBps (MegaBytesPerSecond)
and Mbps (MegaBitsPerSecond). The PCI Bus can transfer data at 132MBps
(That's 132 MegaBytesPerSecond, 32bit transfer not 8 bits). The USB 2.0
specs indicate that the data transfer is 480Mbps (MegaBitsPerSecond). As the
USB interface is a SERIAL device, and the PCI Bus is a Parallel Device.

So, due to the USB 2.0 device being able to transfer data at 480Mbps
(480Mbps/10=48MBps) it will actually transfer data at 48MBps rate, which is
well within the PCI Bus specs. (Not accounting to other devices on the Bus).

The calculation above is mainly based on serial transfers (10Bits) 1 start,
8 data, 1 stop = 10 bits. So dividing the MegaBitsPerSecond by 10, will give
you the parallel transfer rate.

So in conclusion, the USB 2.0 spec is well within the PCI Bus specs, and
will run as specified.

My clarification above is NOT 100%, but it will give people an idea how it
all supposed to work.

The principles above can also be used with network cards too. They too are
serial devices.

Hope this helps.

Wolf

|-----Original Message-----
|From: Jack Bowling [mailto:jbinpg shaw ca] 
|Sent: Monday, 29 July 2002 09:50 am
|To: valhalla-list redhat com
|Subject: Re: USB 2.0
|
|
|** Reply to message from Chris Kloiber <ckloiber ckloiber com> 
|on Sun, 28 Jul 2002 18:03:59 -0400
|
|
|> On Sun, 2002-07-28 at 15:04, Jack Bowling wrote:
|> > I have a USB 2.0 capable scanner and am thrilled that the 2.4.19 
|> > kernel will have support included for this protocol.
|> > 
|> > I have a basic question: I know there are add-in PCI cards to add 
|> > USB 2.0 capabilities to your PC. But can one expect to achieve the 
|> > rated USB 2.0 speed from one of these cards, or would you have to 
|> > have a motherboard that provides USB 2.0 capabilities natively?
|> 
|> I haven't looked at it myself, but is USB 2.0's rated speed faster 
|> than the PCI bus can handle? If not, is it faster than the 
|PCI bus can 
|> handle while other things are happening on the PCI bus? If so, then 
|> it's just a "my number is bigger than yours" game.
|
|Thanks for rephrasing my question, Chris.  USB 2.0 top speed 
|is 480 Mbps. Here is a cut from the PCI specs that has me 
|believing that it should be possible to attain this with PCI 
|add-on cards:
|
|------
|
|3.0 PCI Bus Protocol
|
|PCI is a synchronous bus architecture with all data transfers 
|being performed relative to a system clock (CLK). The initial 
|PCI specification permitted a maximum clock rate of 33 MHz 
|allowing one bus transfer to be performed every 30 
|nanoseconds. Later, Revision 2.1 of the PCI specification 
|extended the bus definition to support operation at 66 MHz, 
|but the vast majority of today's personal computers continue 
|to implement a PCI bus that runs at a maximum speed of 33 MHz.
|
|PCI implements a 32-bit multiplexed Address and Data bus 
|(AD[31:0]). It architects a means of supporting a 64-bit data 
|bus through a longer connector slot, but most of today's 
|personal computers support only 32-bit data transfers through 
|the base 32-bit PCI connector. At 33 MHz, a 32-bit slot 
|supports a maximum data transfer rate of 132 MBytes/sec, and a 
|64-bit slot supports 264 MBytes/sec. 
|
|-----
|
|132 MBps = 1056 Mbps so things should be OK as long as 
|something else isn't saturating the PCI bus.
|
|jb
|
|-- 
|Jack Bowling

######################################################################
Attention: 
This e-mail message is privileged and confidential. If you are not the 
intended recipient please delete the message and notify the sender. 
Any views or opinions presented are solely those of the author.

This e-mail has been scanned and cleared by MailMarshal 
www.marshalsoftware.com
######################################################################





[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]