Modbus simulator is a very nice work that I just try.
But i have a drawback with the number of register.
There is a difference of 1 between my plc register number and Modbus simulator register number.
example (Modbus TCP):
With my PC Application I read Register 41600 in my plc.
To find the same value if I use Modbus simulator, I must read Register 41601.
my two cents