Solution. Using 32 bits per way instead of 16. No need for Robot PLC.
Manual for FD does not include this command. Manual for CFDs (my controller) includes this.
This solution includes sign bit. Start number 65 is an example of a bit area. 32 is the number of bits.
'PLC value (DINT) to Robot Integer
V%[1] = ISIG2INT(65, 32)
'Robot Integer to PLC (DINT)
INT2OSIG O65, 32, V%[1]