Space Before the Physical Unit

If you type a numeric literal with a physical unit, there should be a space between the number and the unit.

    T := 25ns;  -- ILLEGAL, but accepted by ModelSim
    T := 25 ns; -- OK; according to VHDL language standard

Mentor Graphics’ ModelSim and QuestaSim accept the former (illegal) version. As a result, some VHDL designers got used to writing the incorrect version, producing code that is not portable to other simulators. Sigasi Studio accepts the ModelSim-style physical literals but warns about this (rule 47).