64 bit modbus

We need to develop a 64 bit signed datatype for our ModBus master product to support the ModBus 64 bit Signed datatype used in our PM5000 series meters, Can you tell me the bit order, ie MSB first, High word first or what?
Ans
 The INT64 data type uses the MSB format. When reading 4 int16 registers you must use A+B+C+D to compose the int 64 register.