When using a USB-to-Serial interface everything works as long as I don't go beyond 57600 Baud. At higher rates I only get giberish like this:
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What makes the problem so strange is, I exchanged every component and the problem still presists. I tried different OSes (Ubuntu, Windows XP, Windows 7, Mac OS X 10.7) with 32 and 64 Bit. I tried USB-to-Serial interface from FTDI and Prolific. I tried reading the output from my Raspberry PI and from an Asterisk Appliance. I changed the cables and the wiring. Nothing helped.
I made a example with a old notebook with native COM and put the USB-to-Serial to the same connection as "sniffer" (only Rx and GND connected) to make sure the output and everything is ok as one can see on the native port. The voltage is okay. Settings for both are 115200 Baud, 8 Bit with 1 Stop and no flow control. Native is okay. USB is messed up.
I used the newest drivers and double checked all connections. I have no idea what is wrong here. As I couldn't find anyone describing problems like this I question my long experience in computer science and think I'm doing some completely wrong.