Comments by aattanayake (Top 3 by date)

I found the answer and got it working ....

From: http://msdn.microsoft.com/en-us/library/y2sxhat8.aspx

By default, SerialPort uses ASCIIEncoding to encode the characters. ASCIIEncoding encodes all characters greater then 127 as (char)63 or '?'. To support additional characters in that range, set Encoding to UTF8Encoding, UTF32Encoding, or UnicodeEncoding.

my bad.
here is what happening:
when I use VB6 chr() function I get a reply from the connected device. The COM port analyzer reads the following string from the VB6 app 2B 04 03 E8 00 02 F6 71.

When I use the ToChar() in VB.NET there is no reply from the other devise.
The analyzer reads 2B 04 03 3F 00 02 3F 71 from the serial port (note the 3F, like you said it must be converting the 8 bit value dropping the rest)

As you suggested when I use the ToByte() function I still don't get a reply from the device. And the analyzer reads 34 34 30 32 33 32 30 32 32 34 36 31 31 33

Can you think of any thing that might fix my problem?
Thanks