Wright, Robert B. wrote:
> i didn't realize that SCSI interfaces posed a problem.
> what kind of latencies are expected with ethernet and scsi?
Ethernet: collisions (causes retransmission) and transmission errors may occur.
SCSI: seeking may be extended by mechanical vibrations or read errors
(causes re-read) may occur.
Ok, you may give a definit Worst-Case-Timing for the cases described above,
but theses timings are >10 times higher than the likely timings.
Consequence: the system will be much too oversized -> costs increasing.
So people usualy take the likely timings a get in account that
sometimes deadlines are missed ... (missing data frames etc. = soft-RT)
Mostly, the incoming/outgoing data-rate is much lower than
the HD-transferrate, so that an hard-RT awareness of harddisks is
not necessary (Data buffered in memory).
If you wish to have a better RT-aware ethernet-communication, then
use point-to-point full-duplex connections with directly
programming the ethernet-chip. I took a look into the (standard)Linux
source-code for ethernet low-level drivers: it should not be
too much effort to convert them to RT-Linux drivers.
(So ethernet is used on ISO-OSI level 2 instead of ISO-OSI level 4 with
TCP/IP)
Comments?
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