Please Help with IDE serial plotter and monitor on UNO R4 Minima

samdug · October 6, 2026, 12:33pm

I cannot get the serial plotter or the serial monitor to show anything when running a sketch on the R4 Minima which is a genuine part purchased from Mouser last month.

I have searched the forum and elsewhere and tried some suggested fixes.

I am using the Arduino IDE:

Version: 2.3.10
Date: 2026-06-09T16:48:46.639Z
CLI Version: 1.5.1

I reinstalled this today but it did not help.

Uno Rev4 minima connected to ACER Aspire AV15-51 Laptop using a USB type C lead.

The program compiles and uploads successfully

I am running the following program.

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  // I tried some suggested fixes found on the internet
  //while(!Serial) { ; } // Stops program from running
  delay(1500); // Makes no difference apart from delaying the start
}

void loop() {
  
  Serial.print("myVariable:");
  Serial.print(myVariable);
  Serial.print(",");
  Serial.print("Constant:");
  Serial.println(static_variable);

  ++myVariable;
  if(myVariable> 10) myVariable = 1; // keep the count between 1 and 10
  if (!(myVariable % 2)) digitalWrite(LED_BUILTIN, LOW); // turn on LED if myVariable is odd
  else digitalWrite(LED_BUILTIN, HIGH); // turn off LED if myVariable is even
  
  delay(100*myVariable); // pause for between 100ms and 1 second
}

The program is definitely running. The built-in LED is flashing as required by the program.

Nothing shows on either the serial monitor or serial plotter. I have tried many different programs to test the plotter, including examples from the Arduino Forum.

Please see the attached screenshot for details.

I have used a UNO Rev 3 for several years without any problems, but consider myself to be a novice. any help would be most appreciated. Thank you for reading this!

P.S. I just added another suggested fix from this forum, before the Serial.begin() line, as follows:

  while (!Serial && (millis()) < 5000) {;};
  Serial.begin(9600);

This did not solve the problem.

xfpd · October 6, 2026, 1:34pm

These need to be declared.

int myVariable = 42;
int static_variable = 3;

samdug · October 6, 2026, 1:37pm

Multiple apologies. I had declared them, but I failed to copy the first 2 lines of my program>

int myVariable = 1;
int static_variable = 500;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  while (!Serial && (millis()) < 5000) {;};
  Serial.begin(9600);
  // I tried some suggested fixes found on the internet
  //while(!Serial) { ; } // Stops program from running
  delay(1500); // Makes no difference apart from delaying the start
}

void loop() {
  
  Serial.print("myVariable:");
  Serial.print(myVariable);
  Serial.print(",");
  Serial.print("Constant:");
  Serial.println(static_variable);

  ++myVariable;
  if(myVariable> 10) myVariable = 1; // keep the count between 1 and 10
  if (!(myVariable % 2)) digitalWrite(LED_BUILTIN, LOW); // turn on LED if myVariable is odd
  else digitalWrite(LED_BUILTIN, HIGH); // turn off LED if myVariable is even
  
  delay(100*myVariable); // pause for between 100ms and 1 second
}

xfpd · October 6, 2026, 2:30pm

Do you have this selected for plotter?

samdug · October 6, 2026, 2:36pm

Many thanks for your help.

Yes I tried that, but in the picture I included, I was trying the monitor.

here it is with the plotter. The LED is flashing as expected.

xfpd · October 6, 2026, 2:42pm

Your graph ranges from -0.2 to 1.2. Is that the expected range?

Perhaps this "no print" is that the lines are "background" color (multiple lines should give multiple colors).

OR

this is similar to other topics where they see no code text, but can see sidebar activity of the cursor.

Also... other posts suggest the R4 Serial Port needs up to 5000 ms to start.

samdug · October 6, 2026, 3:33pm

Thanks again.

I have put the 5000 ms delay in at the beginning of the setup()

while (!Serial && (millis()) < 5000) {;};

The expected range is 1 to 10. In all my previous uses of the serial plotter, the scale changed to fit the size of the variable being transmitted and the colours sorted themselves out. I am not aware of any method of fixing them.

I am beginning to think that there is a fault in the firmware on the board, but have not yet found a reliable source for a method to update it or check it.

samdug · October 6, 2026, 3:36pm

Also I can't see any cursor activity in the plotter or the monitor.

JohnLincoln · October 6, 2026, 5:43pm

I tested your code from post #3 on my Arduino UNO Minima, and got the following results:

If you un-tick the 'Constant' box at the top left, then the vertical axis re-scales so that you can see 'myVariable' in greater detail:

Your code works satisfactorily for me, so there must be some problem with your setup.

xfpd · October 6, 2026, 5:49pm

When you "click" in the code section of the IDE do you see "over/under" lines (orange in my case) where the cursor should be (blinking white, vertical, pipe in my case - not visible in the screen capture)?

Can you see the background change when you "highlight" code text?

JohnLincoln · October 6, 2026, 6:11pm

I'd never seen those orange lines before, so I did some investigation.
I changed the 'theme' in 'preferences' and found out the following:

The orange lines are only there if you have set the theme to 'Dark High Contrast'.
If you have the theme set to 'Light High Contrast' then the lines are there but a different colour (grey?).
If your theme is 'Light' or 'Dark' then there are no lines.

I can see from samdug's screenshots that he is using the 'Light' theme, so the lines shouldn't be present.

samdug · October 6, 2026, 11:02pm

Many thanks johnLincoln, for confirming that the code does what I intended. All I can confirm is that the LED flashes as expected.

My setup works fine with an uno rev3. The only difference in the setup, apart from using R4 minima in place of R3, is that the R4 requires a type c usb cable.

I have ordered a new cable in case there is a problem with it. It works fine for uploading code and reading the board info, so it seems odd that it won't work for serial comms. I fear that it may be a problem with the board. I will be getting an R4 Wifi fairly soon, so I will try that in my setup.

Very frustrating as I am totally incapable of testing and debugging code without serial monitor or plotter.

samdug · October 6, 2026, 11:11pm

Many thanks xfpd.

Yes I can now see the cursor blinking and I can highlight text.

It looks like there is a problem with my board or my setup.

xfpd · October 7, 2026, 12:13am

Recently, some users of the IDE have reported something similar, where they did not see text or cursor. Maybe @ptillisch has instructions for you to gather information for Arduino so this issue can be addressed for a permanent resolution, rather than fixing one person's issues.

camsysca · October 7, 2026, 12:37am

samdug:

P.S. I just added another suggested fix from this forum, before the Serial.begin() line, as follows:
  while (!Serial && (millis()) < 5000) {;};
  Serial.begin(9600); 
This did not solve the problem.

No idea why it would, due to incorrect order; to prevent future issues what you want is:

  Serial.begin(9600); 
  while (!Serial && (millis()) < 5000) {;};

Why? Because begin starts initialization of the Serial construct, which can take not-insignificant time in some processor setups; the subsequent test on Serial is monitoring completion of the initialization, so that no attempt is made to print before the Serial system is ready for it.

A further simplification:

  Serial.begin(9600); 
  while (!Serial);

would suffice, as if Serial doesn't complete initialization, the code simply stalls, which is a big hint.

samdug · October 7, 2026, 8:42am

Thanks camsysca, Good point!

I did try while(!Serial) { ; } after Serial.begin(9600); (Now commented out, see beginning of setup() ) and indeed the code stalled, as you said.

samdug · October 7, 2026, 8:50am

Thanks again, xfpd.

ptillisch · October 7, 2026, 9:15am

Hi @samdug. I'm going to ask you to provide some additional information that might help us to identify the problem.

This procedure is not intended to solve the problem. The purpose is to gather more information.

Please do this:

Select File > Quit from the Arduino IDE menus if it is running.
All Arduino IDE windows will close.
Start Windows "File Explorer".
Open the Arduino IDE installation folder.
ⓘ The default installation location is at one of the following paths:
- ```
C:\Program Files\Arduino IDE
```
- ```
C:\Users\<username>\AppData\Local\Programs\Arduino IDE
```
  (where <username> is your Windows username)
  - Note that the AppData folder is hidden by default. On Windows "File Explorer", you can make it visible by opening the "View" menu, then checking the box next to "☐ Hidden items".
In the folder listing of the Arduino IDE installation folder, hold the Shift key while clicking the right hand button on the mouse.
A context menu will open.
Select "Open PowerShell window here" from the menu.
Windows PowerShell will now open with the current directory set to the Arduino IDE installation folder.
Type the following command in the PowerShell window:
```
& ".\Arduino IDE"
```
Press the Enter key.
Debug output should now be printed to the PowerShell window as Arduino IDE starts.
Wait for Arduino IDE to finish starting.
Connect your UNO R4 Minima board to your computer with a USB cable (if it isn't already).
Select the port of the UNO R4 Minima board from Arduino IDE's Tools > Port menu (if it isn't already).
Open Serial Monitor (if it isn't already).
Switch back to the PowerShell window.
Press the Ctrl+Shift+A keyboard shortcut.
This will select all the text in the PowerShell window.
Press the Ctrl+C keyboard shortcut.
This will copy the contents of the PowerShell window to the clipboard.
Open a reply here on this forum topic by clicking the "Reply" button.
Click the <CODE/> icon on the post composer toolbar.
This will add the forum's code block markup (```) to your reply to make sure the error messages are correctly formatted.

Code block icon on toolbar390×118 4.25 KB
Press the Ctrl+V keyboard shortcut.
This will paste the output into the code block.
Move the cursor outside of the code block markup before you add any additional text to your reply.
Click the "Reply" button to publish the post.

Please let us know if you have any questions or problems while following those instructions.

samdug · October 7, 2026, 3:00pm

Thanks ptillisch. I am happy to do that!

I am wondering which issue you are trying to gather more info on. xfpd wrote "Recently, some users of the IDE have reported something similar, where they did not see text or cursor." I do NOT have this issue. My issue is that the serial plotter and monitor do not work using the IDE with the uno rev4 minima board.

I won't get a chance to look at this today, but will reply again as soon as I can.

Best wishes

Sam

xfpd · October 7, 2026, 4:36pm

It seemed that way to me.

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