Robotiq gripper connected to my pc but can't communicate using modbus rtu protocal

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my pc system is ubuntu14.04, ros version is indigo, robotiq 2f-85 gripper is connected to my pc using one rs485 to usb cable, and I used robotiq package to control my gripper using this command

rosrun robotiq_c_model_control CModelRtuNode.py /dev/ttyUSB0

however I received

AttributeError: 'ModbusIOException' object has no attribute 'getRegister'

I checked and found the issue is caused by this line of comModbusRtu.py in /home/username/catkin_ws/src/robotiq/robotiq_modbus_rtu/

response = self.client.read_holding_registers(0x07D0, numRegs, unit=0x0009)

this line throws 'ModbusIOExcption'.

I thought my gripper isn't connected to my pc properly, so I checked my usb port withdmesg | grep tty & lsusb and get

[2483608.973947] usb 1-13: FTDI USB Serial Device converter now attached to ttyUSB0
Bus 001 Device 005: ID 0403:6015 Future Technology Devices International, Ltd Bridge(I2C/SPI/UART/FIFO)

It seems that my pc already detects the gripper. The LED in gripper is always red, however when I connect the RS485 to usb cable of gripper to ur robot controller, the LED turns blue and I can control it using polyscope.

I can't figure out where the problem lies, can anybody help me?