Hello Giuseppe,
DO command provides you with a number whose binary representation provides the state of the DOUTs. So the correct command is ?DO, without any argument.
An example to make it more clear:
if ?DO returns 8320 then:
8320 = 0010 0000 1000 0000
So starting from the LSbit you see the state of each digital output. With 8320 DOUT1 is 0 and DOUT2 is 0.
If it was 8321 = 0010 0000 1000 0001 then DOUT1 would be 1.