Thanks for helping me. The picture is taken while the manufacterers programm is communicating with the Heating unit. Upper-left is the cable to the CHU. It has only 4 wires: +5V, 0V, and RX/TX. Right-below is the USB/TTL converter. In the middle is the RS232-TTL converer (MAX232A). This communication works fine.
When I try the UNO communicate with the CHU I connect the RX/TX wires from the UNO to the MAX232 (marked with small black and red dots below the MAX232). The signals of the UNO show up at the cable to the CHU (which I can see on my scope). The CHU does not respond as I described earlier while it does so when I send the same data by means of the RealTerm programm. When I parrallel to the output from the MAX232 connect a serial cable to COM1 of the PC the sniffer (DeviceMonitor.exe) sees no data until I also start the RealTerm programm. Then the sniffer says: "port opened" and shows the data from the UNO. The RealTerm Programm on its own always sees the UNO data.
I hope this additional information helps.
