Thanks for all the replies.
Reply to Przemek Klowoski
=========================
I agree it is almost certainly a serial buffer overflow somewhere. By
filling and emptying the buffers between my code on the BBB and my code on
the PC I can detect a 12KB buffer. I believe that this 12KB buffer is in
the BBB driver on the PC. I tried to set the driver's buffer to be bigger
but the driver appears to ignore all calls to the WINAPI SetupComm system
call.
I have tried OSX but I still get data loss.
I have tried to use a second BeagleBone as PC replacement but the first
beagle bone does not show up on the list of available serial ports.
I have tried:
x = serial.Serial('COM10', xonxoff=True)
x = serial.Serial('COM10', rtscts=True)
x = serial.Serial('COM10', dsrdtr=True)
None of these cause any change in behaviour.
I have also tried sending stop commands directly:
x.write(b'\x13') # byte 0x13 is XOFF
This does not cause any change in behaviour either.
Reply to arsi
=============
If I understand you correctly, you are recommending adding a 4 byte
sequence "0x10 0x11 0x12 0x13" on a regular basis in the streaming data to
allow the receiving PC to sync up with the stream. Is that what you mean?
For my current application the data looks like:
10392.61424 125 1299
10392.61469 125 1242
10392.61514 125 1357
10392.61558 125 1385
10392.61628 125 1357
10392.61674 125 1242
10392.61718 125 1328
10392.61765 125 1473
10392.61810 125 1212
10392.61880 125 1299
It is delimited by "/r/n" which allows easy syncing up. The trouble is that
bytes can go missing to make the above data look like:
10392.61424 125 1299
10392.61469 125 1242
10392.61514 125 1357
10392.61558 125 1385
103921628 125 1357
10392.61674 125 1242
10392.61718 125 1328
10392.61765 125 1473
10392.61810 125 1212
10392.61880 125 1299
I could make my format checking stricter and I could add checksums to every
line. This would cut down the error rate but I would prefer to fix the
cause of the data loss if possible.
Reply to Graham
===============
I have tried baud rates on the PC from 10Hz to 100GHz including common baud
rates like 9600. The serial connection works at all baud rates tested.
There appears to be no change in the data rate or the error rate. To rule
out a bug in the pyserial library I have also changed the baud rate with a
GUI terminal called "Termite 3.1". This also does not change the data rate.
I believe that the serial port is a virtual device and all commands to
change the baud rate are ignored by the driver.
x = serial.Serial('COM10', baudrate=10)
x = serial.Serial('COM10', baudrate=9600)
x = serial.Serial('COM10', baudrate=100_000_000_000)
My current ideas
=============
I could add checksums. I could switch from a virtual serial port to using a
pair of physical USB-RS232 adapters. This would go: BBB USB->hardware
RS232->PC USB. I only mention this because I have plenty of experience with
physical RS232 and I have USB-RS232 adapters which respond to the win32
SetupComm call to change the buffer size. I could also switch to something
else entirely like SSH over the ethernet port.
--
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