C#

//required to use Stopwatch
using System.Diagnostics;

// inside Form Class scope
private bool hasResponded = false;

private Stopwatch theStopWatch;

// Stopwatch delay in milliseconds #1
private int waitHowLong1 = 250;

// Stopwatch delay in milliseconds #2
private int waitHowLong2 = 3000;

// original code missing parameter Type ! 
private void SerialDataReceivedEventHandler(dynamic port_DataReceived)
{
   // whatever ... use try/catch ?

   hasResponded = true;
}

private void senddata1(String str)
{
    // Convert the user's string of hex digits (ex: B4 CA E2) to a byte array
    //byte[] data = HexStringToByteArray(str);
    
    hasResponded = false;
    
    // Send the binary data out the port
    //comport.Write(data, 0, data.Length);
    
    // create and start a new Stopwatch
    theStopWatch = Stopwatch.StartNew();
    
    // delay 250 milliseconds
    while(true)
    {
        if (theStopWatch.ElapsedMilliseconds >= waitHowLong1) break;
    }
    
    Console.WriteLine("250 ms. wait done");
    
    theStopWatch.Stop();
    
    theStopWatch.Reset();
    
    theStopWatch.Start();
    
    // how long should you wait before you decide the update has failed ?
    while (true)
    {
        if (theStopWatch.ElapsedMilliseconds >= waitHowLong2) break;
    }
    
    theStopWatch.Stop();
    
    Console.WriteLine("3000 ms. wait done");
    
    if (! hasResponded) Console.WriteLine("failed to handle update");
}