simple RD6006 interactions via modbus RTU

USAGE:

rd6006_op.pl [options]

options:
[-h] -- help;
[-d] -- debug;
[-m /dev/device] -- device file, default =  /dev/ttyUSB0;
[-u unit] -- modbus device unit, default=1;
[-O=off] -- off power after mesurement;
[-P=on] -- on power before mesurement;
[-v v_set] -- initial voltage set;
[ -i i_set ] -- initial current set;
[-V d_v] -- voltage delta on every iteration;
[-I d_i] -- currtnt delta on every iteration;
[-n n_read] -- number of iterations;
[-t t_read] -- additional delay between iterations.