Ivan,
You will probably have to play tricks, I don't think that the modbus registers will take a "negative" number in a 16 bit register.
so, in your PLC or what ever....
You will need to push your unsigned number into the register that represents the value.
then you will need to turn on a another "bit" (that can be the register value following the one above).
in your PLC, if your number is NEGATIVE, turn on your "Negative Control Bit" (or just push a "1" into your second register).
Then in Brains:
Bring in your number value from the modbus and also look at the second register if it is NOT on, send the value to your dro.
second rung, do the same, but if it IS ON send the value through a formula = (value * -1) then to your dro, to give you a negative value in the dro.
scott