How do I force a serial port write method to wait for the line to clear before sending its data?

Question

Here's some background on what I'm trying to do:

1. Open a serial port from a mobile device to a Bluetooth printer.
2. Send an EPL/2 form to the Bluetooth printer, so that it understands how to treat the data it is about to receive.
3. Once the form has been received, send some data to the printer which will be printed on label stock.
4. Repeat step 3 as many times as necessary for each label to be printed.

Step 2 only happens the first time, since the form does not need to precede each label. My issue is that when I send the form, if I send the label data too quickly it will not print. Sometimes I get "Bluetooth Failure: Radio Non-Operational" printed on the label instead of the data I sent.

I have found a way around the issue by doing the following:

for (int attempt = 0; attempt < 3; attempt++)
{
    try
    {
        serialPort.Write(labelData);
        break;
    }
    catch (TimeoutException ex)
    {
        // Log info or display info based on ex.Message
        Thread.Sleep(3000);
    }
}

So basically, I can catch a TimeoutException and retry the write method after waiting a certain amount of time (three seconds seems to work all the time, but any less and it seems to throw the exception every attempt). After three attempts I just assume the serial port has something wrong and let the user know.

This way seems to work ok, but I'm sure there's a better way to handle this. There are a few properties in the SerialPort class that I think I need to use, but I can't really find any good documentation or examples of how to use them. I've tried playing around with some of the properties, but none of them seem to do what I'm trying to achieve.

Here's a list of the properties I have played with:

• CDHolding
• CtsHolding
• DsrHolding
• DtrEnable
• Handshake
• RtsEnable

I'm sure some combination of these will handle what I'm trying to do more gracefully.

I'm using C# (2.0 framework), a Zebra QL 220+ Bluetooth printer and a windows Mobile 6 handheld device, if that makes any difference for solutions.

Any suggestions would be appreciated.

[UPDATE]

I should also note that the mobile device is using Bluetooth 2.0, whereas the printer is only at version 1.1. I'm assuming the speed difference is what's causing the printer to lag behind in receiving the data.

Answer 1

Well I've found a way to do this based on the two suggestions already given. I need to set up my serial port object with the following:

serialPort.Handshake = Handshake.RequestToSendXOnXOff;
serialPort.WriteTimeout = 10000; // Could use a lower value here.

Then I just need to do the write call:

serialPort.Write(labelData);

Since the Zebra printer supports software flow control, it will send an XOff value to the mobile device when the buffer is nearly full. This causes the mobile device to wait for an XOn value to be sent from the printer, effectively notifying the mobile device that it can continue transmitting.

By setting the write time out property, I'm giving a total time allowed for the transmission before a write timeout exception is thrown. You would still want to catch the write timeout, as I had done in my sample code in the question. However, it wouldn't be necessary to loop 3 (or an arbitrary amount of) times, trying to write each time, since the software flow control would start and stop the serial port write transmission.