which works fine when tested in this environment (sub functions 00 to
03).
Since I don't have the industrial environment available for testing
purposes I want to
do the tests by means of two PCs. And although I use a proper cable
(switched lines
RxD and TxD and so on) [in German: "Null-Modem" ... sorry, I don't know
the English
name for it] but I get a timeout-error regardless of sending or
receiving data.
Since I kow that this works (in some environments at least), my question
is why do I get
a timeout error? Where should I look for the cause of the error?
Cabling, software,
interrupt calls?

Thanks for any help!

I really do not understand why the inventors of the PC found it
clever to handle the keyboard with a ROM BIOS interrupt and a
typeahead queue, but did not handle the COM ports in a similar
manner. They were obviously office type guys or even worse.
Windows at least has a proper driver for the COM ports. And
DR-Multiuser DOS has it too, for up to 64 ports on demand. It
is my preferred OS, look on my homepage for REAL/32.