FP said:
Don't know if this suitable place to ask this Q, but here goes:
I have a Pentium 3 with mainboard that only has USB 1.1 sockets. I have
a couple of USB2 devices, a scanner and flash card reader.
The man at the local computer shop says that if I install a USB 2.0 PCI
card, I won't get USB 2.0 performance from these devices because PCI
slots only work at 33 (or is it 66?) MHz and USB2 is 480 Mbps.
He said to get full USB2 performance I'll have to upgrade mainboard, CPU
and memory.
True or false?
PCI is 33MHz times 32 bits bus. That equals 1056Mb/sec aggregate bandwidth.
USB2 is 480Mb/sec. Both busses have overhead that prevents them from
reaching their full potential, so the same ratio will exist between
them in practice.
Looks like even the lowliest PCI option is faster than the USB2,
so your PCI bus can handle at least one USB2 card.
The USB2 transfer protocol requires some processor intervention, so
the time between data burst on the cable could depend on the
processor performance. If it takes the processor as much time to
set up a packet transfer, as the transfer itself, then the max
rate on the USB2 cable will be cut in half. But at least it won't
be a bus limitation. No matter what processor/mobo is used, nobody
ever gets the full 480 megabits per second, due to processing
overhead.
Another limitation can be the bandwidth available in the memory
subsystem. Sometimes the PCI bus can be limited by how much
bandwidth there is available between Northbridge and Southbridge,
and by how fast you can pump data into main memory. There have
also been some chipsets that have had abysmal PCI transfer
performance. Generally speaking, a chipset that uses something
faster than PCI, to connect the Northbridge to the Southbridge,
won't have a PCI issue.
The worst motherboard I've ever heard of (AMD chipset, with a bug in
its PCI architecture) was only capable of 25MB/sec, instead of the
100MB/sec practical transfer rate you can get with most current
boards. That bug had to do with the fact the chipset supported two
PCI busses, and the fat bus got priority over the skinny one.
If you were using a motherboard with that chipset on it, you would
definitely not be getting full USB performance from one of its
32 bit slots.
Hope that gives you some idea as to what to expect,
Paul
