IEEE 1394

[Joshua Halpern]

Eugen Leitl wrote:
SNIP....
> "The IEEE 1394 bus has a minimum latency of a few hundred microseconds and 
> a worst-case delay of a few milliseconds. For large data blocks, this bus 
> uses direct memory access (DMA) similar to PCI bus mastering that reduces 
> the influence of software protocol overhead on the transfer rate. The 
> 400-Mb/s top data rate supports consumer digital video equipment and data 
> acquisition devices requiring relatively fast data transfer. Bus latencies 
> are compared in   Figure 1 and bus throughput in   Figure 2."

Although it is not relevant to Beowulfs, firewire
is being touted here as a replacement of IEEE-488.
They may have missed the boat, the movement in
the measurement community is to USB-2.0

People who do real time data acquisition have less
trouble with IEEE-488 than with the insane way it
is implemented in various devices, the belt and
suspenders handshaking required to even start
talking to devices (remember the design
was for an electrically noisy environment) and the
fact that the device hardware implementation often
has more than a lot to be desired.  The bus itself
may only have a ms of latency much of which is due
to the handshaking, but the time to rip something
out of a device may be a tenth of a second  or more
in the worst (Tektronix) case.  You can sometimes
be indirect and clever, but not always and the cost
in your time is humongous.

None of these issues are really addressed by the
new buses as you find out when you talk to the
manufacturers and ask how many measurements can
I do per second and they tell you what the transfer
rate is when it finally squirts out of the narrow
end of the device.

Josh Halpern