A DTR line is used to signal that the host has configured a terminal and
is ready to transmit and receive data from the USB CDC/Serial device.
When this test suite is run with the use of a Linux host, a workaround has
to be used to overcome some platform specific DTR line behavior.
Every time the serial port file descriptor is opened, the DTR line is
asserted until the terminal attributes are set.
As a consequence, the device receives a premature DTR signal with a
duration of 200-500 us before the correct, long-lasting DTR signal set by
the host-side test script. (tested on the Linux kernel 4.15.0)
The solution is to wait for the first DTR spike, ignore it, and wait for
the correct DTR signal again.
Despite having ECHO and ECHOCTL POSIX lflags disabled by default by
pyserial, a delay is needed for host to properly handle data received
from USB serial/CDC device.
With no delay host would echo 0x00-0x31 characters prefixed with '^'.
Filip Jagodzinski
Russ Butler