real, dimension(100) :: b
integer(int8), dimension(100) :: a
call random_number(b)
a = b*256 - 128
Fortran has no "byte" type per se, but it has (if supported) an integer type with 8 bits, which has the kind from iso_fortran_env. The intrinsic subroutine random_number returns random numbers in the range of [0..1[.