Python string formatting to send through serial port

Question

I need to properly format the string in order to send it to the arduino connected through a serial port. For example I have this python2.7.5 code:

x = int(7)
y = int(7000.523)
self.ser.write("%s%s" % (x, y))

but I want x in a byte and y in different bytes from x so I can assign a variable for each recieved byte in the arduino code similar to this:

for (i=0; i<3; i++) 
  {
   bufferArray[i] = Serial.read();
  } 
d1 = bufferArray[0];
d2 = bufferArray[1];
d3 = bufferArray[2];
x = d1;
y = (d2 << 8) + d3;

In other words, I don't want that a piece of y is in the x byte. What is the proper string format to do this?

I think what you are looking for is the struct module. It'll help you pack/unpack data as you like. — Mattias Nilsson
– Mattias Nilsson, Commented Aug 4, 2013 at 10:31

nio · Accepted Answer · 2013-08-04 14:04:47Z

1

Following the advice of @Mattias Nilsson there is a sample code if you want to send two consecutive 16 bit unsigned integers:

import struct
x = int(7)
y = int(7000.523)
buf = struct.pack("<HH", x, y)
# read it back
for i in buf:
    print "%02x" % (ord(i))

You can see that they are send each in 2 bytes and the LSB byte is always first. (Tested on intel x64 machine python 2.7.5) Edit: You should be able to explicitly set the endiannes using the < character for little endian order at the beginning of the format string.

Then you could just send both buffer and the string using Serial.write:

self.ser.write(buf+yourstring+'\0')

You can nottice the zero charater that will terminate your string. If you send the string like this you should not send any zero byte character in your string.

On the arduino side you should read and decode those two integers first and then to read characters in a loop that will end reading if you read a zero byte. You should definitely check if your reading buffer won't overflow too.

edited Aug 4, 2013 at 14:04

answered Aug 4, 2013 at 10:39

nio

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3 Comments

mBolz Over a year ago

Thank you very much, but I need to send the x and y in the same serial.write comand and also to send them as a single string. Is there a way to avoid using the "for" cycle?

nio Over a year ago

you can try those suggestions, i would be happy to hear if it worked out and, if I helped you could accept my answer later.

Sylvain Leroux Over a year ago

@user2650205 From serial-line point of view, there is no such thing as a "string". Serial line only read and write bytes, one at a time. Sending several bytes in a for loop or using ser.write(string) does not change anything.

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