Thank you for your reply.
Searching for drivers was one of the first steps I did. But unfortunately my device is so old (about 25 years) that there is no driver. I also tried to work with the programs of the LabVIEW LLB which are available from your link, but there I also get the 1073807339 error code.
Checking the cabling is for sure a good idea. So far it seems to be fine.
Thank you for your hint that the use of delays and the Bytes at Serial Port is incorrect.
But why is Bytes at Port then mentioned in this NI-document:
http://digital.ni.com/public.nsf/allkb/874B379E24C0A0D686256FCF007A6EA0
There they also mention the delay which should be used for older instruments. Since my device is about 25 years old, I thought that the use of wait is a good idea.
But perhaps you are right. So, could you please correct my program then? As I mentioned, I have never worked so far with the RS232 interface, so I really would be glad if somebody corrects my code.