var sw = new Stopwatch();
sw.Start();

// Write the "request" bytes.
_port.Write(buffer, 0, buffer.Length);

Console.WriteLine("After write: {0}", sw.ElapsedMilliseconds);
sw.Restart();

// Wait for response, but timeout after 100ms.
var timeoutCount = 0;
while (_port.BytesToRead == 0 && timeoutCount < 100)
{
    Thread.Sleep(1);
    timeoutCount++;
}

Console.WriteLine("Waited: {0}, #loops: {1}", sw.ElapsedMilliseconds, timeoutCount);

if (_port.BytesToRead == 0)
{
    // Timed-out..
    return null;
}

// Read the response.
var receivedData = new byte[_port.BytesToRead];
_port.Read(receivedData, 0, receivedData.Length);

return receivedData;

After write: 0
Waited: 14, #loops: 1
After write: 0
Waited: 14, #loops: 1
After write: 0
Waited: 14, #loops: 1
...and so on...

After write: 0
Waited: 4, #loops: 5
After write: 0
Waited: 2, #loops: 3
After write: 0
Waited: 2, #loops: 3
After write: 0
Waited: 1, #loops: 2
After write: 0
Waited: 3, #loops: 4