# PySerial not sending string to serial port of Arduino

**URL:** <https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696>\
**Category:** Python Help\
**Created:** [October 4, 2022, 10:49pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696 "2022-10-04T22:49:28Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 4, 2022, 10:49pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/1 "2022-10-04T22:49:28Z")

</div>

Hello. I’m kind of new to Python and Arduino but I started to work recently with these two, so I need some help.

This is the scenario:  
**At work:**  
We are working with an Arduino Uno and a Rpi 4. They are conencted via USB cable and we connect to the Rpi 4 via work Wireless network.  
We use Windows computers, so we are connecting to Rpi 4 via Putty.  
The Arduino Uno has a code that is prepared to send a JSON object to its serial port when it catches a string “getData” on its serial port! So, my colleague asked me to write a script to automatically send this string and fetch the JSON object into a variable.  
At work I was able to get the JSON object in Putty even if completely out of format, but I got it, using 2 separate scripts (goal is to use only one). I don’t have a screenshot of the result from work, though!

**At home** :  
On my free time, I’m also trying to do the same at home, but I use Debian instead of Windows and therefore, I use minicom instead of Putty. Other than that, I just wrote a small program to try to mimic the program my colleague uploaded on Arudino at work. Meaning that if I send the string “getData” to Arduino serial port via minicom, I get the JSON object back at minicom.  
Something like this:  
 ![image](https://us1.discourse-cdn.com/flex002/uploads/python1/original/2X/0/0865d2cfd4b743f2fa8336c63534ac0f357b05c9.png)  
We don’t see the string “getData” here because I deactivated `local Echo` in minicom.  
If I type something else I just get a “Not detected” message.

So, the goal is, with a single script, to mimic this behavior. Send “getData” to Arduino serial, wait for the JSON object, fetch it into a variable and print it out to the terminal window.

So far, I was not able to reproduce this behaviour with a single script, so I tried to split the script in 2 so that I could run both at the same time. I don’t want to detail all the things I tried because the post will become huge and nobody will read it.

So, I have this code for now, in a single script:

```python
import serial

ser = serial.Serial(
        port = '/dev/ttyUSB0',
        baudrate = 115200,
        timeout = 10,
        xonxoff = False
)
ser.reset_output_buffer()
ser.reset_input_buffer()
val = 0
command = ''
command = input("Enter command: ")
val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
print("Waiting bytes: ", ser.in_waiting)
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(in_data)

```

This is the result, after the `timeout` of 10 secs:

```auto
$ python3 serialWriteRead.py 
Enter command: getData
Waiting bytes: 0
Bytes written: 7
b''

```

The problem is that I can’t even catch the “getData” string reaching the **Rx** line of Arduino on my scope (at home), using the script! I can’t also catch it if I use minicom where I can see the Tx (??) led blinking per each charater of the string “getData” that I type in minicom window. I can only catch the JSON object reply when using minicom, on my scope!  
When I added the (??) above, I meant that I was expecting to see the string “getData” on the Rx line of the Arduino and the JSON object on the **Tx** line of the Arduino. What I see is, while using minicom, is the opposite: “getData” on **Tx** line and the JSON object on the **Rx** line. But while using the script above, I only see **Tx** line blinking. I assume is on the `ser.write()` command! No **Rx** line at all!

So, what can I do/change to try to make this work?  
Thanks  
Psy

---

<div class="post-metadata">

**Author:** ![cameron](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/cameron/32/2658_2.png) [@cameron](https://discuss.python.org/u/cameron)\
**Post date:** [October 4, 2022, 11:31pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/2 "2022-10-04T23:31:59Z")

</div>

> This is the scenario:  
> **At work:**  
> We are working with an Arduino Uno and a Rpi 4. They are conencted via USB cable and we connect to the Rpi 4 via work Wireless network.  
> We use Windows computers, so we are connecting to Rpi 4 via Putty.

I’m assuming this is Windows-(wifi/ssh)-\>Rpi-(serial)-\>Arduino.

> The Arduino Uno has a code that is prepared to send a JSON object to its serial port when it catches a string “getData” on its serial port! So, my colleague asked me to write a script to automatically send this string and fetch the JSON object into a variable.  
> At work I was able to get the JSON object in Putty even if completely out of format, but I got it, using 2 separate scripts (goal is to use only one). I don’t have a screenshot of the result from work, though!

We _much_ prefer plain text inline to screenshots. You’re dealing with  
text, so you can copy/paste from the putty window’s text. Just place it  
here between triple backticks as you’ve doing with your Python code  
(without the `python` annotication), eg:

````
 ```
 pasted output
 goes here
 ```

````

With that we can copy/paste the text ourselves, and people with visual  
impairment can read it readily in their preferred form.

> **At home** :  
> On my free time, I’m also trying to do the same at home, but I use Debian instead of Windows and therefore, I use minicom instead of Putty.

Wouldn’t it be neater to just `ssh` from your debian box to the Rpi? Or  
are you using the Rpi’s serial console? Not very important though.

> Other than that, I just wrote a small program to try to mimic the  
> program my colleague uploaded on Arudino at work. Meaning that if I  
> send the string “getData” to Arduino serial port via minicom, I get the  
> JSON object back at minicom.  
> Something like this:  
> ![image](https://us1.discourse-cdn.com/flex002/uploads/python1/original/2X/0/0865d2cfd4b743f2fa8336c63534ac0f357b05c9.png)

Also, those of us on email don’t see these. I had to duck out to the  
forum to see your screenshot, and the important bits are just text.

> We don’t see the string “getData” here because I deactivated `local Echo` in minicom.  
> If I type something else I just get a “Not detected” message.
> 
> So, the goal is, with a single script, to mimic this behavior. Send “getData” to Arduino serial, wait for the JSON object, fetch it into a variable and print it out to the terminal window.
> 
> So far, I was not able to reproduce this behaviour with a single script, so I tried to split the script in 2 so that I could run both at the same time. I don’t want to detail all the things I tried because the post will become huge and nobody will read it.
> 
> So, I have this code for now, in a single script:
> 
> ```python
> import serial
> 
> ser = serial.Serial(
> port = '/dev/ttyUSB0',
> baudrate = 115200,
> timeout = 10,
> xonxoff = False
> )
> ser.reset_output_buffer()
> ser.reset_input_buffer()
> val = 0
> command = ''
> command = input("Enter command: ")
> val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
> print("Waiting bytes: ", ser.in_waiting)
> print("Bytes written: ", val)
> in_data = ''
> in_data = ser.read_until(b'}')
> print(in_data)
> 
> ```
> 
> This is the result, after the `timeout` of 10 secs:
> 
> ```auto
> $ python3 serialWriteRead.py
> Enter command: getData
> Waiting bytes: 0
> Bytes written: 7
> b''
> 
> ```
> 
> The problem is that I can’t even catch the “getData” string reaching the **Rx** line of Arduino on my scope (at home), using the script! I can’t also catch it if I use minicom where I can see the Tx (??) led blinking per each charater of the string “getData” that I type in minicom window. I can only catch the JSON object reply when using minicom, on my scope!  
> When I added the (??) above, I meant that I was expecting to see the  
> string “getData” on the Rx line of the Arduino and the JSON object on  
> the **Tx** line of the Arduino. What I see is, while using minicom, is  
> the opposite: “getData” on **Tx** line and the JSON object on the  
> **Rx** line. But while using the script above, I only see **Tx** line  
> blinking. I assume is on the `ser.write()` command! No **Rx** line at  
> all!

I’m assuming your setup is Debian-(serial)-\>Rpi-(serial)-\>Arduino.

You’re using minicom to talk to the Rpi, yes? I see in your screenshot  
that minicom itself is using `ttyUSB0`. I assume that is your Debian  
box’s `ttyUSB0`. But the implication is that at the Rpi end it is  
connected to a serial port as well. Which one?

And the Arduino is presumably also connected to an Rpi serial port.

Your Rpi programme is opening `ttyUSB0` expecting to talk to the Arduino  
device. _Is_ the Arduino connected to `tyUSB0` on the Rpi? Can you check  
that?

Might `ttyUSB0` on the Rpi be the same device you’re using for the Deian  
host?

The fact that you do not see the LEDs flickering on your scope suggests  
that the Rpi programme is sending data to the wrong serial port,  
something other than the port talking to the Arduino.

Can you check that? Can you experiment with other `port` devices on the  
Rpi to see if they activate your scope’s LEDs?

Finally, I don’t see any login prompts etc in your screenshot. Are you  
going Debian-(serial)-\>Arduino with no Rpi?

Cheers,  
Cameron Simpson [cs@cskk.id.au](mailto:cs@cskk.id.au)

---

<div class="post-metadata">

**Author:** ![MRAB](https://avatars.discourse-cdn.com/v4/letter/m/e9c0ed/32.png) [@MRAB](https://discuss.python.org/u/MRAB)\
**Post date:** [October 5, 2022, 2:01am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/3 "2022-10-05T02:01:10Z")

</div>

When you’re using minicom or putty, you’re typing the command followed by end-of-line (LF or CRLF) and all that is being sent and it flushes the output.  
When you write using `ser.write`, there’s no end-of-line and no explicit `ser.flush()` either.  
I suspect that it’s all still in the output buffer.

---

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**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 5, 2022, 9:47am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/4 "2022-10-05T09:47:00Z")

</div>

> [@cameron](#):
>
> I’m assuming this is Windows-(wifi/ssh)-\>Rpi-(serial)-\>Arduino.

Exacty that!

> [@cameron](#):
>
> We _much_ prefer plain text inline to screenshots. You’re dealing with  
> text, so you can copy/paste from the putty window’s text.

I sometimes use screenshots more because of the importance of showing the position of the mouse cursor after some action due to the differences that there are betwen Windows an Linux handling `\r` and `\n` characters. But ok, when I’m at work, I’ll use copy/paste. It’s just that sometimes is important to see where the cursor mouse is to understand if we have an extra `\r` or `\n` where it shouldn’t be.

> [@cameron](#):
>
> Wouldn’t it be neater to just `ssh` from your debian box to the Rpi? Or  
> are you using the Rpi’s serial console? Not very important though.

You mean to be able to work on the Rpi 4 and the (work) Arduino instead of trying to mimic the situation at home? If it is this what you mean, I could do it but I would need to get the ssh keys to be able to login from home (we setup RSA key pairs to access the Rpi 4 at work because I think we had already 2 ransomware attempts on MongoDB we have in Rpi 4 before).

> [@cameron](#):
>
> I’m assuming your setup is Debian-(serial)-\>Rpi-(serial)-\>Arduino.

At home no, I’m using my laptop if it was the Rpi 4. So, setup is: laptop-Debian\<-\>(USB|serial)\<-\>Arduino.

> [@cameron](#):
>
> You’re using minicom to talk to the Rpi, yes?

From the previous reply, no. I use minicom to talk directly to my (home) Arduino. The only difference from home to work is the Wireless and Rpi 4. I’ll try to picture the 2 situations one next to the other:

**Work** :  
Laptop → ssh to Rpi (via Wireless) → Arduino (via minicom) – I use this option  
or  
Laptop → ssh to Rpi (via Wireless) → Arduino (via Aruino Serial Monitor) – my colleague uses this option

**Home**  
Laptop → Arduino (via minicom)

> [@cameron](#):
>
> And the Arduino is presumably also connected to an Rpi serial port.
> 
> Your Rpi programme is opening `ttyUSB0` expecting to talk to the Arduino  
> device. _Is_ the Arduino connected to `tyUSB0` on the Rpi? Can you check  
> that?
> 
> Might `ttyUSB0` on the Rpi be the same device you’re using for the Deian  
> host?

So, at home, my laptop is as if it was the Rpi 4 at work.  
I’m not sure but I think Rpi 4 somehow sees Arduino on it’s `ttyACM0` port and at home, my laptop sees my Arduino at `ttyUSB0`. I’m not sure this is a physical thing or a software level thing, because I use Debian at home and the Rpi 4 at work is using Ubuntu.

> [@cameron](#):
>
> The fact that you do not see the LEDs flickering on your scope suggests  
> that the Rpi programme is sending data to the wrong serial port,  
> something other than the port talking to the Arduino.
> 
> Can you check that? Can you experiment with other `port` devices on the  
> Rpi to see if they activate your scope’s LEDs?

At this moment I can only test things at home. No Rpi 4 at home, unfortunately.

> [@cameron](#):
>
> Finally, I don’t see any login prompts etc in your screenshot. Are you  
> going Debian-(serial)-\>Arduino with no Rpi?

Yes, at home I’m not using Rpi 4. I’m really sorry, it’s clear I wasn’t clear enough in my opening post about the setups. I hope I was better now clearing out the scenarios!

> [@MRAB](#):
>
> When you’re using minicom or putty, you’re typing the command followed by end-of-line (LF or CRLF) and all that is being sent and it flushes the output.  
> When you write using `ser.write`, there’s no end-of-line and no explicit `ser.flush()` either.  
> I suspect that it’s all still in the output buffer.

Hum, so I need to explicitely send the `\n` along with the string? Like `getData\n` ? Is that what you mean?

Also, from the docs, the `Serial.flushOutput()` seems to have been renamed to [serial.reset\_output\_buffer()](https://pyserial.readthedocs.io/en/latest/pyserial_api.html#serial.Serial.reset_output_buffer). Am I wrong??? They say there ` Changed in version 3.0: renamed from flushOutput()`.

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 5, 2022, 1:42pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/5 "2022-10-05T13:42:50Z")

</div>

Well, peeps, I finally overcome all obstacles with the help of a friend and the guys in IRC channel! I was able to pick up all stuff in Rx and Tx lines, and get the entire JSON response as I wanted.

I had 2 problems. One was that at some point in time, I saved one of the scripts with the wrong name (serialWrte.py instead of serialWrite.py) and I was running a different file from what I was editing. The other problem was that I needed to explicitely send the `\n` char along with the `getData` string and I wasn’t doing so.  
At this point I’m getting what I want!

Next Thursday I’ll test the script at work and I hope it works flawlessly despite the differences of the scenario, otherwise I’ll be back here.

Here is the single script working:

```python
import serial

ser = serial.Serial(
        port = '/dev/ttyUSB0',
        baudrate = 115200,
        timeout = 10,
        xonxoff = False
)

val = 0
command = ''
command = input("Enter command: ")
command += str('\n')
val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(in_data)

```

I know I was sugegsted to change the terminating char of the JSON object but at this point I’m not sure I can do that because I’m not the “owner” of the object. I don’t know if adding any other char at the end of the JSON object isn’t going to break somehting else. I’ll see when I have the chance to talk to my work colleague!

Thanks all for now!

---

<div class="post-metadata">

**Author:** ![MRAB](https://avatars.discourse-cdn.com/v4/letter/m/e9c0ed/32.png) [@MRAB](https://discuss.python.org/u/MRAB)\
**Post date:** [October 5, 2022, 5:03pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/6 "2022-10-05T17:03:27Z")

</div>

The `.write` method writes only the bytes that you give it. If you were writing a block of binary data, you wouldn’t want an additional 0x0A to be added at the end, would you? 🙂  
Only the `print` function adds a line ending.  
The `.flush()` method that I mentioned will send the contents of the output buffer immediately; that’s different from the `.reset_output_buffer()` method that will _discard_ the contents of the output buffer.

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 5, 2022, 6:31pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/7 "2022-10-05T18:31:16Z")

</div>

I’m not sure I understand why would I need the `flush()` command. What does `flush()` do that `Serial.write()` don’t?

Also, I thought you were referring to `flushOutput()`. This one was renamed recently, iiuc.

---

<div class="post-metadata">

**Author:** ![cameron](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/cameron/32/2658_2.png) [@cameron](https://discuss.python.org/u/cameron)\
**Post date:** [October 5, 2022, 11:38pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/8 "2022-10-05T23:38:27Z")

</div>

> I’m not sure I understand why would I need the `flush()` command. What  
> does `flush()` do that `Serial.write()` don’t?

Nothing, if I’m reading the docs correctly:  
[https://pythonhosted.org/pyserial/pyserial\_api.html#serial.Serial.write](https://pythonhosted.org/pyserial/pyserial_api.html#serial.Serial.write)

It exists for compatibility with normal Python “filelike” objects, in  
that you can pass a `Serial` object to functions expecting to write to  
files. But a `Serial` object is effectively an _unbuffered_ file in this  
use.

But this interface lets you go, for exqmple:

```
 print("blah", file=serial)

```

Now, `print()` will just write what you tell it to a file (default  
`sys.stdout`), and _not flush_. Flushing is at the discretion of the  
file object itself. Data going to a regular file is typically flushed  
(_written_) when the buffer fills, and a tty file such as your terminal  
is typically set to flush when a newline is seen (or the buffer files).

The `print()` function s not asking to flush the buffer - the data might  
land in the file’s buffer and go nowhere (yet). Calling `.flush()` on  
the file _forces_ the file to write the data now. `print()` has an  
optional `flush` parameter, default `False`, to call `flush()` on the  
file.

The `Serial.flush()` method exists so that people using it as a file can  
call `flush`. But because the `Serial` object is unbuffered (the `write`  
method really writes to the serial port) it has no effect. Other than to  
no break code expecting to be able to call `flush`.

> Also, I thought you were referring to `flushOutput()`. This one was  
> renamed recently, iiuc.

Yes, to the less confusingly named (and more conventionally snake\_cased)  
`reset_output_buffer`. `reset_input_buffer` and `reset_output_buffer`  
exist to _discard_ the contents of the input and output buffers, and the  
latter aborts any pending write. I expect they’re to aid resetting your  
state because you’ve lost track, been interrupted (eg by the user) or  
are otherwise trying to start afresh with something.

Cheers,  
Cameron Simpson [cs@cskk.id.au](mailto:cs@cskk.id.au)

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 6, 2022, 9:28am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/9 "2022-10-06T09:28:20Z")

</div>

Yesterday after I get this working I still add another feature, which was to beautify the JSON object sent by Arduino, using `json` library, by adding the following line at the end of the code:

```python
print(json.dumps(json.loads(in_data.decode()), indent = 4))

```

At home I was able to get the JSON object properly formatted.

Today, I brought my own Arduino clone (DCcEle DCcduino UNO) to work to make sure the code I wrote at home and worked, would also work here… Guess what? It doesn’t.

From a Windows machine, directly connected to my Arduino clone, the code that yesterday worked at home, doesn’t work at work, with my own Arduino.  
This is the code:

```python
import serial, json
 
ser = serial.Serial(
        port = '/dev/ttyUSB0',
        baudrate = 115200,
        timeout = 10,
        xonxoff = False
)
val = 0
command = ''
command = input("Enter command: ")
command += str('\n')
val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(in_data)
print(json.dumps(json.loads(in_data.decode()), indent = 4))

```

I get this error:

```auto
>python serialWriteRead.py
Enter command: getData
Bytes written: 8
b'Detected{"Temperature": 25,"Pressure": 11,"Light": 1200}'
Traceback (most recent call last):
  File "C:\Users\cecol\Documents\Arduino\Arduino-Home\serialWriteRead.py", line 18, in <module>
    print(json.dumps(json.loads(in_data.decode()), indent = 4))
  File "C:\Users\cecol\AppData\Local\Programs\Python\Python310\lib\json\ __init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "C:\Users\cecol\AppData\Local\Programs\Python\Python310\lib\json\decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "C:\Users\cecol\AppData\Local\Programs\Python\Python310\lib\json\decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

```

If you guys can help me figure out what is wrong, I would appreciate!  
Is this the common problem about the different way of Windows and Linux handling CR and LF chars?

---

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**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 6, 2022, 2:58pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/10 "2022-10-06T14:58:46Z")

</div>

Ok, nevermind peeps, it’s all working.  
At work I didn’t know the baudrate on the Arduino was different from what I was using at home.  
The script works and pretty prints the JSON object just perfectly!

Thanks every one  
Psy

---

<div class="post-metadata">

**Author:** ![cameron](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/cameron/32/2658_2.png) [@cameron](https://discuss.python.org/u/cameron)\
**Post date:** [October 6, 2022, 9:56pm UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/11 "2022-10-06T21:56:25Z")

</div>

You might also want to try:

```
 from pprint import pprint
 pprint(json.loads(in_data.decode()))

```

which is also nicer to read. The output is Python syntax rather than  
JSON, but sometimes that’s nice.

Cheers,  
Cameron Simpson [cs@cskk.id.au](mailto:cs@cskk.id.au)

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 11, 2022, 10:33am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/12 "2022-10-11T10:33:20Z")

</div>

Well, after all I need to make a small change and this change is breaking the script again.  
Instead of requesting the user the command to send to serial port, I need to send it hardcoded!

So, I tried several changes but they all hang up the script:

```python
import serial, json

ser = serial.Serial(
        port = '/dev/ttyACM0',
        baudrate = 9600,
        timeout = 10,
        xonxoff = False
)
val = 0
#command = ''
#command = input("Enter command: ")
#command = "getData"
#command += str('\n')
val = ser.write("getData".encode(encoding = 'ascii', errors = 'strict'))
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(json.dumps(json.loads(in_data.decode()), indent = 4))

```

I almost can bet that this is the missing or extra `\n` char or something related.

I also tried:

```python
val = ser.write("getData\n".encode(encoding = 'ascii', errors = 'strict'))

```

```python
val = ser.write('getData\n'.encode(encoding = 'ascii', errors = 'strict'))

```

```python
val = ser.write('getData'.encode(encoding = 'ascii', errors = 'strict'))

```

and also tried to do something like:

```python
command = ''
#command = input("Enter command: ")
command = "getData"
command += str('\n')
val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(json.dumps(json.loads(in_data.decode()), indent = 4))

```

or,

```python
command = ''
#command = input("Enter command: ")
command = 'getData'
command += str('\n')
val = ser.write(command.encode(encoding = 'ascii', errors = 'strict'))
print("Bytes written: ", val)
in_data = ''
in_data = ser.read_until(b'}')
print(json.dumps(json.loads(in_data.decode()), indent = 4))

```

But they all hang, and eventually throw out something like:

```auto
d$ python3 serialReadWrite.py
Bytes written: 8
Traceback (most recent call last):
  File "serialReadWrite.py", line 18, in <module>
    print(json.dumps(json.loads(in_data.decode()), indent = 4))
  File "/usr/lib/python3.8/json/ __init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

```

What am I missing’

---

<div class="post-metadata">

**Author:** ![abessman](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/abessman/32/8367_2.png) [@abessman](https://discuss.python.org/u/abessman)\
**Post date:** [October 11, 2022, 10:49am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/13 "2022-10-11T10:49:09Z")

</div>

> [@PsySc0rpi0n](#):
>
> I also tried:
> 
> ```python
> val = ser.write("getData\n".encode(encoding = 'ascii', errors = 'strict'))
> 
> ```
> 
> ```python
> val = ser.write('getData\n'.encode(encoding = 'ascii', errors = 'strict'))
> 
> ```

These will both produce identical output as your previous user-entered command:

```python
>>> command1 = "getData\n".encode(encoding = 'ascii', errors = 'strict')
>>> command1
b'getData\n'
>>> command2 = input("Enter command: ")
Enter command: getData
>>> command2 += str('\n')
>>> command2 = command2.encode(encoding = 'ascii', errors = 'strict')
b'getData\n'
>>> command1 == command2
True

```

Are you _certain_ the script works if you take the command as user input instead of hardcoding it? I suspect something else is wrong.

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 11, 2022, 11:05am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/14 "2022-10-11T11:05:41Z")

</div>

Yes, I’m sure. I will paste this screenshot to show both the code running and the output:

 ![](https://us1.discourse-cdn.com/flex002/uploads/python1/original/2X/8/8548ac147b9a883968d23ebd03c23caff3231393.png)

---

<div class="post-metadata">

**Author:** ![abessman](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/abessman/32/8367_2.png) [@abessman](https://discuss.python.org/u/abessman)\
**Post date:** [October 11, 2022, 11:10am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/15 "2022-10-11T11:10:19Z")

</div>

Mysterious. Perhaps your terminal is adding extra characters to your input string? Could you add `print(repr(command)`) after the call to `serial.write` and run the user input version of the script again? What does it output?

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 11, 2022, 11:16am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/16 "2022-10-11T11:16:10Z")

</div>

The only difference I see is the `b'` thing indicating bytw, iirc.

 ![](https://us1.discourse-cdn.com/flex002/uploads/python1/original/2X/8/8fc8460f55f6c9021f2340c35d2d97332691df6f.png)  
The contents of `command` before sending it to the serial port is:

```python
b'getData\n'

```

After I see:

```python
'getData\n'

```

---

<div class="post-metadata">

**Author:** ![abessman](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/abessman/32/8367_2.png) [@abessman](https://discuss.python.org/u/abessman)\
**Post date:** [October 11, 2022, 11:26am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/17 "2022-10-11T11:26:43Z")

</div>

> [@PsySc0rpi0n](#):
>
> The contents of `command` before sending it to the serial port is:
> 
> ```python
> b'getData\n'
> 
> ```
> 
> After I see:
> 
> ```python
> 'getData\n'
> 
> ```

The contents of `command` do not change when you write it to the port. It’s always `"getData\n"`. Encoding it turns it into `bytes`, which is represented as `b'getData\n'`, but you do not store the encoded command.

Either way, that does not seem to have been the issue.

I’m stumped, to be honest. Some weird race condition maybe, where waiting for user input allows the serial bus time to set up properly but writing the hardcoded command immediately makes it hang? A long shot, but try importing `time` and add a short `sleep` before the `write` call.

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 11, 2022, 11:31am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/18 "2022-10-11T11:31:31Z")

</div>

Yeah, I already tried that with a 1s delay in place of the `input()` line of code.  
I think I’ll try smaller delay!

---

<div class="post-metadata">

**Author:** ![abessman](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/abessman/32/8367_2.png) [@abessman](https://discuss.python.org/u/abessman)\
**Post date:** [October 11, 2022, 11:46am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/19 "2022-10-11T11:46:59Z")

</div>

Yeah, looks like that might be it: [serial port - pySerial write() works fine in Python interpreter, but not Python script - Stack Overflow](https://stackoverflow.com/questions/28192190/pyserial-write-works-fine-in-python-interpreter-but-not-python-script?rq=1)

Try 5 seconds or so.

---

<div class="post-metadata">

**Author:** ![PsySc0rpi0n](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/psysc0rpi0n/32/9222_2.png) [@PsySc0rpi0n](https://discuss.python.org/u/PsySc0rpi0n)\
**Post date:** [October 11, 2022, 11:53am UTC](https://discuss.python.org/t/pyserial-not-sending-string-to-serial-port-of-arduino/19696/20 "2022-10-11T11:53:52Z")

</div>

The minimum was 1.8 seconds. And it worked!  
Thanks @abessman

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