On 5/22/2012 8:42 AM, Grant Edwards wrote:
On 2012-05-22, Albert van der Horst<alb...@spenarnc.xs4all.nl> wrote:
It is anybody's guess what they do in USB.
They do exactly what they're supposed to regardless of what sort of
bus is used to connect the CPU and the UART (ISA, PCI, PCI-express,
USB, Ethernet, etc.).
If a device is registered as /dev/ttyUSBnn, one would hope that
the Linux USB insertion event handler, which assigns that name,
determined that the device was a serial port emulator. Unfortunately,
the USB standard device classes
(http://www.usb.org/developers/defined_class) don't have "serial port
emulator" as a standardized device. So there's more variation in this
area than in keyboards, mice, or storage devices.
The best answers is probably that it depends on the whim of whoever
implements the usb device.
It does not depend on anybody's whim. The meaning of those parameters
is well-defined.
Certainly this stuff is system dependant,
No, it isn't.
It is, a little. There's a problem with the way Linux does
serial ports. The only speeds allowed are the ones nailed into the
kernel as named constants. This is a holdover from UNIX, which is a
holdover from DEC PDP-11 serial hardware circa mid 1970s, which had
14 standard baud rates encoded in 4 bits. Really.
In the Windows world, the actual baud rate is passed to the
driver. Serial ports on the original IBM PC were loaded with
a clock rate, so DOS worked that way.
This only matters if you need non-standard baud rates. I've
had to deal with that twice, for a SICK LMS LIDAR, (1,000,000 baud)
and 1930s Teletype machines (45.45 baud).
If you need non-standard speeds, see this:
http://www.aetherltd.com/connectingusb.html
If 19,200 baud is enough for you, don't worry about it.
John Nagle
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