Pyserial formatting - bytes over 127 are returned as 2 bytes, not one
I have a program running on my Arduino that takes serial input and stores it in a variable. Works in charm. With Arduino apps embedded in the serial monitor, I have successfully sent and received bytes between 0-255.
Using pyserial to send any byte above 127 (or 0b01111111
) pyserial
returns 2 - the value for values above 127, say 0b10000000
2 bytes will be sent, not one.
I believe my problem is related to pyserial
.
ser.write(chr(int('01000000', base=2)).encode('utf-8'))
works fine and is correctly adopted on Arduino.
ser.write(chr(int('10000000', base=2)).encode('utf-8'))
returns 2, however - and shows how to Arduino 0b11000010
and 0b10000000
.
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As NPE says, this is the encoding for UTF-8 - a byte between 128 and 2047 (8-11 bits) inclusive is converted to two bytes: if the original 11 bits are abcdefghijk, then the utf-8 version is 110abcde 10fghijk. In your example (with padding left 0s to make 11 bits), 00010000000 would be converted to 11000010 10000000 or \ xc2 \ x80, which is exactly what you see. See the Wikipedia article on UTF-8 for more
You can see it in python with this code (I'm replacing int ('10000000', base = 2) with 128):
>>> unichr(128).encode('utf-8')
'\xc2\x80'
What confuses me is that you can use chr (int ('10000000', base = 2)). encode ('utf-8') or equivalent chr (128) .encode ('utf-8) ". When I do this, I get:
>>> chr(int('10000000', base=2)).encode('utf-8')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 0: ordinal not in range(128)
Have you changed the default encoding?
You need an encoding that uses one byte for 0 - 255 and is unicode. So try using "latin_1" instead:
>>> unichr(128).encode('latin_1')
'\x80'
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