John said:
Paul wrote:
[Snip]
Caveats on the NEC chip. It has a PCI Express x1 lane interface on it.
The interface runs at Rev.2 or Rev.1 speeds. If you plug it into a
Rev.1 slot, then the theoretical speed is reduced from the maximum
values you might see listed. There is currently only one
hardware peripheral device right now, that cares about this.
For connecting an external USB3 hard drive, it's not important.
The first motherboards to have predominantly Rev.2 interfaces, are the
Intel Sandy Bridge motherboards (LGA1155). With many older boards, you
have to do careful analysis, to determine your slot type, and whether
that NEC USB3 card you bought, ends up at full speed.
[Snip]
Just been checking the full spec of my Dell Inspiron 580.
The chipset is Intel H57.
The PCI Express x1 slots apparently have a bus speed of 500MB/s
(bidirectional). Is that Rev.2?
We're talking about "the art of deception" here. Some clever
engineers, total the bandwidth from both directions and quote that.
We have nasty swear words we use in the office, amongst ourselves,
reserved for such people.
If I use the H57 datasheet, section 5.2 says:
"5.2 PCI Express* Root Ports (D28:F0,F1,F2,F3,F4,F5, F6, F7)
There are eight root ports available in the PCH. The root ports are compliant to the PCI
Express 2.0 specification running at 2.5 GT/s...
PCI Express Root Ports 1-4 and Ports 5-8 can independently
be configured as four x1s, two x2s, one x2 and 2 x1s, or
one x4 port widths."
Now, if I compare that to a P67 (the chip used with Sandy Bridge)
"5.2 PCI Express* Root Ports (D28:F0,F1,F2,F3,F4,F5, F6, F7)
There are eight root ports available in the PCH. The root ports are compliant to the PCI
Express 2.0 specification running at 5.0 GT/s...
PCI Express Root Ports 1–4 or Ports 5–8 can independently
be configured as four x1s, two x2s, one x2 and two x1s, or
one x4 port widths."
That tells you, your H57 PCI Express x1 lane runs at Rev.1 rates
(250MB/sec in each direction). The P67 runs at Rev.2 rates
(500MB/sec in each direction).
The way the math works, is as follows. A lane is serial. It is
8B10B encoded. That means, 10 bits are sent, and after decoding,
yield 8 usable data bits. If the link on your H57 runs at 2.5GT/sec,
you multiply that by 8/10 to eliminate the overhead bits and
the bandwidth they're taking. That leaves 2Gbit/sec of user data.
Dividing that by 8 bits per byte, gives 250MB/sec transfer rate.
It's bidirectional, so some grocery store accountant type might
call that a "500MB/sec interface", but that's just wrong. In a
typical usage scenario, one direction carries much more data than
the other (a predominant direction), so 250MB/sec is the kind of
information users need to know.
It means, if a USB3 was running from your Southbridge port, it's a
half speed implementation. It's not the end of the world, and
only a BlackMagic video capture box would complain about it.
For transferring files from storage devices, this is still
a fast interface.
My motherboard has similar issues.
CPU
|
Northbridge ------ Video slot (uses 500MB/sec lanes)
| ------ Video slot (uses 500MB/sec lanes)
| DMI
|
Southbridge ------ x1 lanes (uses 250MB/sec lanes)
If I purchase a NEC USB3 card, and use the little x1 slots,
I'd get a half speed result. But since I only use one video
slot, my second video slot can also be used. If I plug a
NEC USB3 card into the second video slot, it runs full speed.
At least, that's the theory. It's an awful waste of a high
speed slot though. Think of the RAID cards I could run in
there instead.
Paul
