Turning Textbox Red with a timer when serial port stops outputting data

Question

I am building a GUI that reads in serial data into a textbox. If there is a problem and the instrument stops working, I want the textbox to turn red. I am trying to do this with a timer, so the timer will read data every 30 seconds, then if there is no data, the timer will then turn the textbox red. However, nothing happens when I unplug the instrument. How can I turn the textbox red when the instrument stops working?

Here is the relevant code:

        private void timer1_Tick(object sender, EventArgs e)
    {
        DateTime timenow = DateTime.Now;
        TimeSpan span = GPSlastdatatime - timenow;
        timespan = Convert.ToDouble(span.TotalSeconds);

        if (timespan > 30)
        {
            TextBox1.BackColor = Color.Red;

        }
    }

Answer 1

I assume "GPSlastdatatime" is set whenever data comes in?

As is, your "timespan" value is going to be negative.

Switch the order of your subtraction line:

TimeSpan span = timenow - GPSlastdatatime;

By the way, you could do it all in one line like this:

timespan = (DateTime.Now - GPSlastdatatime).TotalSeconds;