see rules 6 and 8:
Hyphens' main purpose is to glue words together. They notify the
reader that two or more elements in a sentence are linked. Although
there are rules and customs governing hyphens, there are also
situations when writers must decide whether to add them for clarity.
When used as an adjective, which is the prefered use, with or without
a hyphen? 8bit data bus 8-bit data ... a quick Google and "8-bit" gets
6.4m hits and "8bit" gets 3.2m hits.
Combine the hyphen usage, with the preference of 8-bit over 8bit and you have your rules.
8-bit is much clearer and per the hyphen rules "glues two words together" to make a new compound adjective. The new sentence would then be "8-bit processor" whereby the new compound adjective, 8-bit has been created to describe the noun processor. Incidentally, "eight-bit" is not used because the context is computer science and mathematics. Number use is common and fast, you would never see pie written as "three point one four one five nine" etc. Therefore 32-bit is correct.
As for the other examples, apply the same logic. "Recommend Standard number 232", RS is a descriptive acronym and 232 is the identifying standard, glue the words together to get RS-232.
DB-9 denotes a D-sub with a 25-position shell size and a 9-position contact configuration, DB is commonly used so therefore the compound adjective would be DB-9.
MSB is an acronym by itself and can be used standalone, I/O is Input/Output and so a slash is okay to use. By the time you got to model-checker, you seem to have got yourself confused and are going a bit hyphen mad! Model and checker are separate word that you can lookup individually in the dictionary, you-don't-need-to-hyphen-everything!