Modbus read problem

I have many modbus devices connected to serial RS485/modbus network in my home.
It is simply not possible to scan every register in every device I would run out of steam!
I wish to read selected registers on demand, only when their information is needed.
I can find NO examples of how this can be done in the documentation, all are time polled into a variable.
Does anyone know how I can do a single read of uint16 from within the automation file?
Thank you for attention.