I am looking for more information on running "Serial over USB" from a linux
desktop host to the BBB device. Some information on the web seems out of
date, at least on debian 7.7. other information is very helpful. I am
summarizing here some of what I have learned myself first:
a sending desktop linux can send information to the BBB over
/dev/ttyACM0.
a recipient BBB linux can receive information on /dev/ttyGS0 .
this is part of the g_multi kernel module and thus works out of the
box on debian 7.7.
this can be tested as
bbb# cat /dev/ttyGS0
desktop# echo "hello" > /dev/ttyACM0
and the bbb should now echo "hello". the comm is buffered, although I am
not sure on which side (desktop or bbb). this is obvious from looking at
the desktop immediately after a fresh boot:
desktop# cat /dev/ttyACM0
which will still return the log in information from the boot on the first
use. after the buffer is full, the device blocks and waits.
information about the port settings can be found (and potentially set,
though I don't think anything is needed) with
stty -F /dev/ttyGS0 -a
however, I believe that some of this are just "pretend you are rs232"
wrong. this is because I just wrote a little perl program that sends
1Mbyte into the device and then closes. this takes about 1.5 seconds.
This would suggest a raw speed of about 7 Mbaud, a little bit faster than
the 9.6 Kbaud that stty tells me. I am guessing that the "serial port over
USB" uses the USB 1.1 "full-speed" protocol that caps out at 12 Mbaud. I
believe hi-speed 480 Mbaud connections require block operations.
the serial comm speed is interesting to compare to the usb mass storage
speed. A dd from the desktop host to the mounted BBB mass-storage device
partition over USB produces 21 Mbaud. so, the serial connection is about
1/3 of what the BBB is capable of over hi-speed USB mass storage. the eMMC
limits out at about 70Mbaud local, which is itself about three times the
speed of the mass storage driver over the USB 2.0 connection. (and remember
that USB 2.0 is itself limited to 480Mbaud. I also tried to measure the
speed over the usb0 ethernet with dd and netcat [nc] to see how close this
could get, but I failed.)
hope this helps.
* one question: I have lost some information sent forth and back, which I
believe is due to the bbb issuing (from /var/log/syslog) a
[email protected] time over, scheduling restart
is it possible to force ttyGS0 to always be available, and never to want to
restart?
* I may write a different driver that sits on top of the mass storage
driver and communicates over a small shared storage area. it's a crazy
idea, but it could be faster than serial-over-usb if I know that I will be
dealing in blocks of 512 bytes and relatively easy to debug and synchronize.
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