Unable to use SerialDevice.ReadTimeout in Windows 10 IoT

Question

I'm trying to implement a ModBus master on Windows 10 IoT on a Raspberry Pi 2. I'm using an external USB to RS-232 adapter since the internal serial port is reserved for Kernel Debugging.

Serial port is working. My question is mainly about timeout when reading.

Here is my code:

// Initialization
serialDevice.ReadTimeout = new TimeSpan(0, 0, 0, allowedTimeBetweenBytes);
serialDataReader.InputStreamOptions = InputStreamOptions.Partial;

// Reading
uint bytesRead = await serialDataReader.LoadAsync(MaxBufferSize); // 256
// Now use ReadBytes to get actual bytes

With no bytes awailable at the serial port RX input, I'm expecting the LoadAsync method to return 0 after wait. Unfortunately, it never returns. (Ok, it DOES return after 256 bytes are received, but that is not what I want)

Since ModBus intensively uses timeouts, I am not sure how to implement it. I am not even sure I could do it...

Does anyone already used timeouts on Windows 10 IoT serial ports?

Answer 1

Yeah, I couldn't get this working either. I'm not sure where ReadTimeout is actually used by the SerialDevice class internally. But I did end up getting something working by copying the timeout to a CancellationTokenSource.

You can see it in use in the following example I wrote for an old serial Mettler Toledo PS 60 shipping scale, where device is an instance of SerialDevice. Seems to work in my case, at least.

using (var writer = new DataWriter(device.OutputStream))
{
    writer.WriteString("W\r\n");

    using (var cts = new CancellationTokenSource(device.WriteTimeout))
    {
        await writer.StoreAsync().AsTask(cts.Token);
    }

    writer.DetachStream();
}

using (var reader = new DataReader(device.InputStream))
{
    using (var cts = new CancellationTokenSource(device.ReadTimeout))
    {
        var read = await reader.LoadAsync(12).AsTask(cts.Token);

        if (read >= 12)
        {
            var data = reader.ReadString(12);
            reader.DetachStream();

            return ExtractWeightChangedEventArgs(data);
        }
    }
}