RS232 on LP Delta not responsive

Hello, 

I am having problems with the on board RS232 pins on LP Delta. I am building a system that communicates to a device via RS232 with the use of these pins. I have a LP Alpha and my software works without any problems there, but on Delta there is no response at all. 

The COM port is properly recognised and the software connects to that comport properly, however there is no communication with the instrument. I checked that on two LP Delta and it is the same behaviour, while on two LP Alpha it works right away - the same hardware, the same cables, the same connection - just swapped LP form Alpha to Delta and the RS232 does not respond. 

I do not see any issue with the driver on the Deltas as the COM port is recognised properly. I need to use the Deltas I already have for these instruments but currently I run out of ideas how to debug this. Does anyone have any suggestions what me be the cause of that situation?