Arduino communication with modbus. How read/write certain registers

Hello everyone,

I’m busy with a project to let my arduino UNO work as a Master to read Certain registers from a external device. I’m using a max 485 to link the external sensor to my arduino.

Ii have connected the max 485 to my arduino and tried different liberies, like SimpleModbusManager. But I dont know how to write/read the registers which I need.

Can someone help me?

Thanks in advance

external device: