Why won't the system work with F-PROT?

The problem lies with the anti-virus software F-PROT. This software does not accept input from NUL devices. Using the system in single step mode, the INI file parameter TRNFILE = NUL creates a NULL file.

One work around would be to substitute CON or AUX instead of the NUL device. These are two other system devices. More information can be found about system devices in your operating system documentation.

So in the FSISYS.INI file, under the Data control group, change

TrnFile = NUL 

to

TrnFile = CON (or AUX)