Yes you are right 
Well i just gave it a test run.
By the way, i am assuming Arduino due is Arduino Duemilanove w/ Atmega328?
The code gave compiler error in one place.
modbus_configure(&Serial1, COM_baud, SERIAL_8N2, roomNetworkAddress, COM_TxEnablePin, HOLDING_REGS_SIZE, COM_holdingRegs);
&Serial1 which is the serial port reference