Calculate baudrate from serialport!

Question

Hi,

I've plugged in a USB bluetooth dongle into my computer, and I've made a virtuel seriel commport app - and I can communicate just fine with my other bluetooth device, through serial port profile!

The problem is the baudrate, is I can se from this topic: http://social.msdn.microsoft.com/forums/en-US/vssmartdevicesvbcs/thread/f04f57d5-0508-437b-9780-92c621272729/

It doesn't matter what I set the baudrate to, because it's a virtual connection! So I've tried to calculate the baudrate, sending a long byte-array many times and measure how long time it takes, by using the stopwatch class!

comport.WriteBufferSize = 1024;
sw.Start();
while (i<500)
{
comport.Write(payload, 0, 1024);
i++;
}
sw.Stop();

Where payload is a 1024-byte array!

The elapsed time is ~44700 ms, which gives me a baudrate of:
500*1024*8/44700 = 91.63 kbps

The baudrate of the receiving device is 115.2 kbps, why doesn't this fit with my calculations?

I've tried with different WriteBufferSize's and different count-characters (1024, 512, 256, 64, 32 and 16), and I get these results:

16 -> ~ 9 kbps
32 -> ~ 19 kbps
64 -> ~ 34 kbps
128 -> ~ 68 kbps
256 -> ~ 91 kbps
512 -> ~ 91 kbps
1024 -> ~ 91 kbps

How come it's max with 91 kbps? :(

Vulpes · Answer

Funnily enough one of the first things I wondered was whether you could be sending 9 or 10 bits per byte, but I couldn't make the arithmetic work so I dismissed it. Anyway, I'm pleased you've figured it out now :)

Alexander Malik Kleist · Answer

That's some good thoughts you got :) I really appreciate it! I've figure it out though :) Here's the answer: The baudrate is 115,2 kbps - BUT I'm calculating the datarate!!! The used serial communication configuration is 8-N-1, which means everytime I'm sending a data byte, I'm actually sending 10 bits! 1 start bit, 8 data bits and 1 stop bit. When I'm maxing out, it really are sending 115,2 kbps, which is 115,2 * (8/10) = 92,16 kbps of data. That fits very good with my calculations above. Thank you again for your fast replies :)

Vulpes · Answer

A couple more thoughts. Are you using the default encoding of ASCII for the SerialPort class? If you've changed it to UTF-8 and there are some characters being sent in the 128-255 range, then they'll need two bytes rather than one to encode which will upset your baud rate calculations. Have you tried this on a different machine in case there's some physical limitation on the one you're using now? Other than that, at least you'll be able to show in your report that you've looked into this matter even if an explanation has eluded you.

Alexander Malik Kleist · Answer

Seems strange to me too :( Communication is good :) the other device is receiving data just fine - every single byte is received correctly. It's not that big an issue for me if it's 'only' sending with 91 kbps - but I need some explanation in my report, I'm writing. The whole project is a part of a school project I'm working on.

Vulpes · Answer

In that case, I don't know why it's 'maxing out' at that value. Despite the difference in baud rate, does it seem to be communicating OK with your other device?

Alexander Malik Kleist · Answer

Thank you for your reply :) I've tried with comPort.Write(payload, 0, 2048) and it's still ~91 kbps I've tried with comPort.Write(payload, 0, 4048) and still gets ~91 kbps Any explanation why it max out at ~91 kbps?

Vulpes · Answer

I'd try it with a WriteBufferSize and payload of 2048. Assuming you're using the .NET SerialPort class any figure you set which is lower than 2048 is ignored anyway (see remarks in http://msdn.microsoft.com/en-us/library/system.io.ports.serialport.writebuffersize.aspx ). The baud rate may be a little higher when it can send a full buffer at once.