I've been using the SerialPort class new to the .NET Framework 2.0 to
control a thermal printer, and have encountered some strange behavior,
where the SerialPort class seems to miss or eat the first byte of data
it receives from the device. The port is configured as follows:

BaudRate = 38400
DataBits = 8
Handshake = Handshake.XOnXOff
Parity = Parity.None
Port = COM1
StopBits = StopBits.One

All other values are their default.

I have tried hooking the DataReceived event and reading the data using
a loop similar to the following:

while (port.BytesToRead > 0)
{
    byte b = (byte) port.ReadByte();
    ...
}

I've also tried reading all available data in one go, using the
SerialPort.Read(byte[], int, int) method with the same result. I also
tried using the BeginRead()/EndRead() methods on the
SerialPort.BaseStream property with the same result.

I hooked up a protocol analyzer (FTS SerialTestAsync 2) and verified
that I was being sent the proper data by the device. I set a breakpoint
on the first call to SerialPort.ReadByte(), but it is actually
returning the second byte of data.

The behavior is not consistent in that it doesn't happen every run of
the program. Has anyone seen similar behavior? Any workarounds?

Thanks,

Russell Klenk

Hi,

I haven't seen this (ever), and I used it with lots of different devices.  I
wonder if XonXoff is causing trouble?  Try it without enabling handshaking.
I doubt that flow control is required, but, even if it is, this may give use
more information.

Also, you are reading data one byte at a time.  It would be much more
satisfactory to real all of the available data into an array of bytes (and
to process that array) than to use a loop.  Your code is the slowest way
possible to read data.

Dick

I will try without using the XonXoff handshake, though the printer
documentation recommends that it be used.

I have since switched to reading all available data at once, which
performs better, but I still encounter the behavior where the first
byte is eaten.

Russ

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