We Keep Coding

Differences between serial communication in matlab vs. python 3.7? Sending int values over 128 via python serial to an arduino

I have some matlab functions which I would like to translate into python 3.7. The functions calculate values for joint angles of a little robot from trossenrobotics and send those values via serial port to the robot which is controlled by an arduino board. The board runs a programm from Trossenrobotics, which interpretes the data send via the serial port and reacts accordingly.
I already managed to translate all the functions and they give the same outputs as the matlab functions, but the serial communication just doesn't work.
In matlab fwrite(s, int_value) and fread(s) are used for the communication. The int_values represent a highbyte and a lowbyte of a joint position(0-1024) and are send seperately.
In python I used pyserial and the functions s.write(byte) and s.read().
I converted the int values into bytes with chr(int).encode().
Since I was struggling with my actual objective, I first wanted to abstract it and make it simpler. Now I am just trying to turn on an LED on the arduino for 2 seconds, when a special byte is received and send the same byte back to python.
I noticed that as long as the value I am sending is smaller that 128 it works just fine, but when it's greater it won't work.
I printed the output of chr(255).encode() which is b'\xc3\xbf', what to me looked like it could be the problem.
The I tried using chr(255).encode('charmap') and printed it which gives back b'\xff', what looks right to me, but it still doesn't work for numbers between 128 and 255.
I also noticed, that when I send the data over the terminal
with s.write(chr(115).encode()) It doesn't return a value, but when I use
s.write(chr(255).encode('charmap')) it returns a 1.
Here's my python pogramm:
python
import serial
from time import sleep
port = 'COM4'
baudrate = 38400
s = serial.Serial(port,baudrate)
sleep(3)
m = 115
s.write(chr(m).encode())
while s.in_waiting == 0:
print('##### waiting #####')
sleep(2)
if s.in_waiting > 0:
r = int.from_bytes(s.read(), byteorder = 'big')
print(r)
s.close()
And here's the arduino programm:
C#
void setup() {
pinMode(13, OUTPUT);
digitalWrite(13,LOW);
Serial.begin(38400);
}
void loop() {
if (Serial.available() > 0)
{
if (Serial.read() == 's')
{
digitalWrite(13, HIGH);
Serial.write('s');
delay(2000);
}
}
else
{
digitalWrite(13, LOW);
}
}
My questions would be:
Regarding my primary problem (sending multiple bytes via matlab, python):
1) Does anybody know if there are any fundamental differences between serial communication in matlab and in python which could cause my problems?
Regarding my abstracted problem (sending one bye via python):
2) How can I send values greater than 128 (up to 255) via the serial port?

There is no fundamental difference between Python and Matlab on this regard.
But in your Matlab code it seems (I'm assuming because what you say):
The int_values represent a highbyte and a lowbyte of a joint position(0-1024) and are send seperately.
that you're sending an int16
to be able to fit up to 1024.
I have no idea what you're trying to do with chr but I have the feeling what you need is replace these lines:
m = 115
s.write(chr(m).encode())
With (on Python 3.x):
m=115
s.write(m.to_bytes(2, byteorder="big"))
That would write: b'\x00s', a mix of hex and ASCII, but you should not worry about that, because that is exactly the same as b'\x00\x73'
And if you do, then you can do: b'\x00s'==b'\x00\x73' and you'll get True.

Thanks for your answer! And sorry for the late reply.
I already tried that right at the beginning but always got an exception. It took me a while to figure out why. It was because I was using numpy.uint8() for my integer values.
After I removed it I didn't get any exception but it didn't work either.
I used chr() because it didn't throw an exception with the numpy.uint8() and honestly because I did not know what else to do...
Today I finally found the solution.
Here is the link to where I found it:
arduino.stackexchange.com
Using s.write(struct.pack('>B', int_value) works and seems to be the equivalent to matlabs fwrite(s, int_value).
Sorry If my question didn't make a lot of sense to you, I am just glad I finally figured it out.

Why NodeMCU sends data with unwanted number?

I am trying to send a serial data from NodeMCU to Arduino. I use MicroPython to program. As well as Serial.read on Arduino. I can send and receive successfully. But the problem is the NodeMCU sends data along with number which is not needed. And Arduino receives data along with number. For Example, if I send "Hello" it sends as "Hello5". I understood that the number is nothing but the number of alphabets in the string. How can I remove this?
MicroPython on NodeMCU:
import os
import machine
from machine import UART
uart = UART(0)
import time
while True:
uart.write('1')
Arduino program:
String received;
String msg;
void setup() {
Serial.begin(115200);
attachInterrupt(0, light, FALLING);//When arduino Pin 2 is FALLING from HIGH to LOW, run light procedure!
}
void light() {
Serial.println(msg);
}
void loop()
{
if (Serial.available() > 0){
received = Serial.readStringUntil('\n');
msg = received;
}
}

I just checked the microPython's UART (http://docs.micropython.org/en/latest/wipy/library/machine.UART.html) and Arduino's Serial (https://www.arduino.cc/en/Reference/Serial
), and it seems you're missing one initialization line for UART. UART document states the default baud rate it sets is 9600, and you expect a 115200 on serial receiver. I believe setting the baud rate different on each side will have undefined behavior.
In your python code, could you try uart.init(115200) after uart = UART(0) call (and the default values for the rest seems same as the Serial's expectations on receiver)?
Also, Serial document says that if it can't find the char you define in the readStringUntil(), then it'll try until it times out. So I guess your function call times-out because it won't find an endline ('\n') in the stream, because you didn't inject any.
In addition, although the help documents of the functionality you're using don't state such a thing, if you really always get the number of characters as the first char at the receiver, it might be worthwhile to try using that to your advantage. I wonder if you can try to get that number first, then read that many chars afterwards (at the Arduino receiver site). Here's some code I hope may help (I'm afraid I didn't try using it):
#include <string.h>
char buffer[256]; // buffer to use while reading the Serial
memset(buffer, (char)0, 256); // reset the buffer area to all zeros
void loop()
{
if (Serial.available() > 0){
int count = Serial.read(); // the first byte that shows the num of chars to read after, assuming that this is a 'byte' - which means we can have max 256 chars in the stream
Serial.readBytes(buffer, count);
msg = String(buffer);
}
}

python printing out a array of bytes via serial port

I am kind a new to python and I am trying to print out an array of bytes which I am receiving via a serial interface.
The code Looks like this
print('Teststring')
ref.ser.write('outx=0\n'.encode())
ref.ser.flush()
s = self.ser.read(size=10)
print(s)
I am expecting the printout to look like this
Teststring
INPUT DATA (Input of my bytearray s)
So the behaviour is like I have an external device which I want to send an reply to my write/flush command and receive the data via the serial input and print out the received information on my computer.
Right now the issue is that the computer is not printing out the expected input data.
I guess my problem is that I need to convert the data s to a string or something like that?

Raspberry Pi and SPI interface (Python): how to transfer data?

I'm using the SPI interface on my Raspberry Pi.
I can read the registers of the chip (first 20 registers) attached to SPI0 with the following code:
spi=spidev.SpiDev()
spi.open(0,0)
x=spi.readbytes(20)
k=0
for i in x:
print(k,hex(i),i)
k=k+1
My question is: how do I set the value of a specific register using "spi.xfer"? I have no idea what parameters the xfer(...) function takes.
How do I read a value from a specific register using "spi.xfer"?
Although I do have the original C code; I still cannot decipher how to use xfer(...).
Here is the C code:
https://github.com/doceme/py-spidev/blob/master/spidev_module.c

I believe this line of code is key:
PyArg_ParseTuple(args, "O|IHB:xfer", &list, &speed_hz,
&delay_usecs, &bits_per_word)
The documentation tells us that the formatting string means:
O - a Python object
| - indicates that the following are optional
I - an int
H - a short
B - a char
: - start of function name string (xfer)
Of these, it seems sensible to assume that the list is the data to send, and indeed it seems to be interpreted as a list of bytes to send.
Since SPI tends to send and receive in parallel, futher reading of the code leads me to believe that each value in list will be replaced by the byte that was received when that particular byte was sent. It's weird, but that's SPI for you.

Sending float type data from Arduino to Python

I need to send float data to Arduino from Python and get the same value back. I thought to send some float data from the Arduino first. The data is sent as 4 successive bytes. I'm trying to figure out how to collect these successive bytes and convert it to proper format at the Python end (system end)
Arduino code:
void USART_transmitdouble(double* d)
{
union Sharedblock
{
char part[4];
double data;
} my_block;
my_block.data = *d;
for(int i=0;i<4;++i)
{
USART_send(my_block.part[i]);
}
}
int main()
{
USART_init();
double dble=5.5;
while(1)
{
USART_transmitdouble(&dble);
}
return 0;
}
Python code (system end):
my_ser = serial.Serial('/dev/tty.usbmodemfa131',19200)
while 1:
#a = raw_input('enter a value:')
#my_ser.write(a)
data = my_ser.read(4)
f_data, = struct.unpack('<f',data)
print f_data
#time.sleep(0.5)
Using the struct module as shown in the above code is able to print float values.
50% of the time,the data is printed correctly. However, if I mess with time.sleep() or stop the transmission and restart it, incorrect values are printed out. I think the wrong set of 4 bytes are being unpacked in this case. Any idea on what we can do here?
Any other ideas other than using struct module to send and receive float data to and from Arduino?

Well, the short answer is there's some interaction going on between software and hardware. I'm not sure how you're stopping the transmission. I suspect whatever you're doing is actually stopping the byte being sent mid-byte therefore inject a new byte when you start back up. The time.sleep() part could be that some hardware buffer is getting overflowed and you're losing bytes which causes an alignment offset. Once you start grabbing a few bytes from one float and a few bytes from another you'll start getting the wrong answer.
One thing I've noticed is that you do not have any alignment mechanism. This is often hard to do with a UART because all you can send are bytes. One way would be to send a handshake back and forth. Computer says restart, hardware restarts the connection (stops sending stuff, clears w/e buffers it has, etc) and sends some magic like 0xDEADBEEF. Then the computer can find this 0xDEADBEEF and know where the next message is going to start. You'll still need to be aware of whatever buffers exist in the hardware/OS and take precautions to not overflow them. There are a number of flow control methods ranging for XON/XOFF to actual hardware flow control.

Because this question ranks highly on search engines I have put together a working solution.
WARNING: Unless you need to full floating point precision, convert to a string and send that (either using sprintf or dtostrf, or use Serial.print(value,NumberOfDecimalPlaces) (documentation) ). This is because the following solution a) Wont work for machines of different endianess and b) some of the bytes may be misinterpreted as control characters.
Solution: Get the pointer for the floating point number and then pass it as a byte array to Serial.write().
e.g.
/*
Code to test send_float function
Generates random numbers and sends them over serial
*/
void send_float (float arg)
{
// get access to the float as a byte-array:
byte * data = (byte *) &arg;
// write the data to the serial
Serial.write (data, sizeof (arg));
Serial.println();
}
void setup(){
randomSeed(analogRead(0)); //Generate random number seed from unconnected pin
Serial.begin(9600); //Begin Serial
}
void loop()
{
int v1 = random(300); //Generate two random ints
int v2 = random(300);
float test = ((float) v1)/((float) v2); // Then generate a random float
Serial.print("m"); // Print test variable as string
Serial.print(test,11);
Serial.println();
//print test variable as float
Serial.print("d"); send_float(test);
Serial.flush();
//delay(1000);
}
Then to receive this in python I used your solution, and added a function to compare the the two outputs for verification purposes.
# Module to compare the two numbers and identify and error between sending via float and ASCII
import serial
import struct
ser = serial.Serial('/dev/ttyUSB0', 9600) // Change this line to your port (this is for linux ('COM7' or similar for windows))
while True:
if(ser.inWaiting() > 2):
command = ser.read(1) #read the first byte
if (command == 'm'):
vS = ser.readline()
#
ser.read(1)
data = ser.read(4)
ser.readline()
vF, = struct.unpack('<f',data)
vSf = float(vS)
diff = vF-vSf
if (diff < 0):
diff = 0-diff
if (diff < 1e-11):
diff = 0
print "Str:", vSf, " Fl: ", vF, " Dif:", diff
References:
Sending a floating point number from python to arduino and
How to send float over serial

I don't know Python, however, what is wrong with the Arduino sending the number like this:
value= 1.234;
Serial.println(value);
For the Arduino to receive a float:
#include <stdio.h>
#include <stdlib.h>
void loop() {
char data[10], *end;
char indata;
int i=0;
float value;
while ((indata!=13) & (i<10)) {
if (Serial.available() > 0) {
indata = Serial.read();
data[i] = indata;
i++;
}
}
i-=1;
data[i] = 0; // replace carriage return with 0
value = strtof(data,&end);
}
Note this code is untested although very similar to code I have used in the past.