A freind asked for some help with his TTL to RS-232 converter circuit. I didn't understand the real meaning of RS-232 so I did some reading. I learned that orginally RS-232 was developed for one specific purpose and has been borrowed, modified, etc, and used for all sorts of things for which it wasn't intended.
Armed with a little knowledge I inspected my freind's circuit and determined that it can't possibly output anything resembling RS-232 - with only 5 volts to work with (no charge pump,etc). I also read where circuits just like that were said to function acceptably. Eventually I discovered some coherent explanations that said some computers can handle the 0 volts and 5 volts just as well as proper RS-232. It also said that some can't. A electronics professional shared an explanation about an 'asserted state' - and if the signal is not the 'asserted state' it's the other. Since the RS-232 port is looking for one state or the other this makes sense - though I thought the spirit of RS-232 was to not let any low level of signal mean anything.
So from what I've read I'd say that you'd have to go back to a specific standard if it is referenced but also expect that the difference between the 'standard' and what will actually work could be quite different.