The maximum speed of a pc with normal 16550A UART is 115Kbit/sec.
There are extended UARTS wich go higher as this, but using higher baudrates would make your software unusable on pc's with a standard UART.
For the rest it also depends on cable length, enviroment noise, your µC's speed, ...
what do you want to transmit that needs to be so fast?
Hey i know i didnt ask this question but.
What im your view is the maximum safe speed that should be transmitted with rs232. Is it 9600 or do you think you can safely trasmit more without too much errors over the course of lets say, 5 mins non - stop
my controller can do about 2 million instructions per sec.
9600 baud is a safe speed, you could probabely go a bit higher then that but in most cases there's no need because of µC's small RAM space the data exchange is commonly quite small...
You can compute the real speed of baud rate of your uC, then compare with the baudrate of computer (normally 9600, 19200, 115200..) If the inaccuracy rate is only 1-2%, it's good. There is no safe rate at 9600 or 115200... everything is oki if the inaccuracy is small enough.
no, I don't think so. Solving the problems of noise is another problem and we should not take it into acc here. Because, sometimes you need a high rate of tx and rx, in a noisy invironment. This that case, we may use shorter wires, cover the wires with magnetic tap... In vietnam, once an engineer put his uC into a metal box because of electro-magnetic noises.
The planetarium at the Hong Kong Space Museum has a 450 foot parallel (8 bit) bidirectional data bus - running in the same trough as the 220 VAC (or was it 240 VAC) power wiring! (And running at a 1 MHz rate.) And this was designed back in 1978.
BTW, the initial test-of-concept had the data bus (plain old grey ribbon cable - no screening [shielding]) wrapped around every single overhead flourescent shop lite in the lab, and a couple of hundred feet lying all over the floor, where it got walked on in all kinds of weather. The final cable wasn't shielded either. It's probably still working fine, after 26 years.
So one can run a long 1 MHz serial cable if one needs to. (That's just one bit, where the HKSM data bus is 8 bits in the same cable, which causes additional problems.)