Strange behavior in communication with arduino and Tcl/Tk

Hi All,
My Tcl/Tk code for serial communication with Arduino is working but with a strange behavior.
I send a message through the serial port that calls a function in arduino and returns a message. The problem is that, at the beginning, I need to send the command 2 or 3 times before arduino start to respond. After the second or third attempt, the arduino starts to respond normally.

The Arduino code is: (adapted from Gammon Forum : Electronics : Microprocessors : How to process incoming serial data without blocking)
void setup() {

void processIncomingByte (const byte inByte) {
static char input_line[MAX_INPUT];
static unsigned int input_pos = 0;

switch (inByte)
case '\n': // end of text
input_line[input_pos] = 0; // terminating null byte
// terminator reached! process input_line here ...
process_data (input_line);

   // reset buffer for next time
   input_pos = 0;  

case '\r':   // discard carriage return

  // keep adding if not full ... allow for terminating null byte
  if (input_pos < (MAX_INPUT - 1))
    input_line[input_pos++] = inByte;
}  // end of switch

}// end of processIncomingByte

//Process_data to process incoming serial data after a terminator received
void process_data (char *data) {
cmd = strtok(data, ";");
if ( (strcmp(cmd, "checkConnection") == 0) || (strcmp(cmd, "checkconnection") == 0) ) {
transaction_id = strtok(NULL, ";");
checkConnection( transaction_id );
} else {
Serial.print("#unknown command: ");
} // end of if

//Send message "ACK" to to confirm the connection with the Arduino
void checkConnection(char *transaction_id ) {

void loop() {
// if serial data available, process it
while (Serial.available () > 0)
processIncomingByte (;

And the Tcl/Tk code is:

set serial_device [exec ls {}[glob /dev/ttyACM]]
set baud 9600
set parity n
set data_bit 8
set stop_bit 1
set id_serial_port [open $serial_device r+]
chan configure $id_serial_port -mode "$baud,$parity,$data_bit,$stop_bit"
chan configure $id_serial_port -buffering line
chan configure $id_serial_port -blocking 0
chan configure $id_serial_port -encoding ascii

#To set the procedure (receiveReadout) to be called when serial port is readable
chan event $id_serial_port readable [list [self] receiveReadout $id_serial_port]

And the procedure receiveReadout
proc receiveReadout { id_serial_port } {

if { [eof $id_serial_port ] } {
    puts "Closing $id_serial_port"
    catch { close $id_serial_port }

   #To read the response from Arduino
set serial_return [chan gets $id_serial_port]


And send the command to check the connection:

chan puts $id_serial_port checkConnection

But as I said, to start receiving Arduino messages I need to send this command 2 or 3 times.

Please, any tips on how to resolve this?
Thank you,

Is this an Arduino question? Almost nobody here uses Tkl/Tk. I was forced to use it at work for a while, I feel scarred for life. :slight_smile:

Hello, do yourself a favour and please read How to get the best out of this forum and modify your post accordingly (including code tags and necessary documentation of your ask).

Honestly, I don't know if what I'm looking at is due to the Arduino code or the Tcl code.
If the more experienced ones say that the Arduino code is OK, then I eliminate half the doubt and I will seek help from the Tcl community.
Best Regards,

Thanks Jackson,
I will read it.
Best Regards,

the code you posted is unreadable as it stands and probably does not compile (I don't see MAX_INPUT definition for example)... So does not make it easy to help

Dear Jackson,

This Arduino code is documented in one of my projects:

And here the translation to English by Google Translator:

I got a reply on the Tcl forum and I share it with you as it might be useful for other users:

"Most likely you're seeing the delayed startup of the Arduino. When you perform the open of the serial device per Tcl the Arduino is first reset, then waits about 2 seconds in its boot loader, then starts up and executes your program from its flash memory. On the Tcl side use a "after 2000" right after the open, then proceed to normal communication. This might fix your issue. "

Thank you for your attention.

Best Regards,

I meant the lack of code tags (as explained in the post How to get the best out of this forum ) makes it hard to read...

(e eu posso ler portugues)

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