According to the RS-232 standard, it operates at -12/+12 volt signal
levels. Some people do use RS-232 at lower voltages (and short distances),
but I think realistically  5 volts is probably a good minimum. In any case,
the distance you can achieve decreses as you increase the baud rate. 

You may want to look at RS-422. The communication protocol is the same as
RS-232, but electrically communication occurs over two 'balanced pairs' of
wire (one pair for receive, and the other pair for transmit). RS-422 is
more noise resistant, and good for much longer distances than RS-232. 

A company I worked with several years ago used single 16-pin DIPs to
convert RS-232 <--> RS-422 to allow two RS-232 devices at 9600 baud to be
placed further apart in a building (between two electrical rooms, one on
the 5th floor, the other in the basement). Unfortunately I don't remember
what the chip was or even who makes it, but it was cheap (only about a
dollar in small quantities).

>hi there i seem to recall that RS232 transmissions should be done over short
>cables. since there are only -3/+3v levels, is this true.
>
>Since this transfer shall travel from 0.3 M - 200M i may need a potent
>voltage level and a good shielding.
>
>Which do you think is the best way too boost such a signal, should i boost
>it straight from the output pin from the pic, with the help of a NPN
>transistor, that drains say.. 12v-24v then convert that level of voltage
>back to TTL with a 7805, is the 7805 fast enough? we are after all talking
>1200 baud here, i do not seem to find a datasheet that describes this.
>
>Or make some sort of optic transmission, ( which is less cheap ), via optic
>cables, the positive side of this is that it wont be affected by
>distrubances.
>
>Hit me with some of your experiences in this matter, It would be be most
>appreciated.
>
>
>Greets
>Robert
>
>input...input... i need.. input...
>
>
---
Markku VenŠlŠinen
markkuv@uniserve.com