Automaticly switch and fallback RS232 async/sync ports

Erkan,

What type of failures need to be detected? In other words, How does PC "A" RS232 lines act during a failure? 
Checking for no signal, 0V, from "A" is easy.

I think ,  it does not matter, how to control this failure mode , Anyway I find a signal to control failure or normally mode.

We assumed that ,  Input voltage is,  in normally mode 5v ,  otherwise 0v.

My big problem is how to switch 7 pin (In Db9) and11 pin (DB25) , which component  , for example  TS12A12511 is it good choice, İS TS3L501ERUAR usable,  how to reduce voltage level  etc.

Thanks

Erkan,

The RS232 signals can be as high as +/-12V so the analog switch would need to be capable this voltage.
I was thinking of using three 3.3V/5V RS232 devices to multiplex (and buffer) the modem lines.
Do you want to switch all 8 data lines or a lesser amount?

Thanks,

I havent any request for all 8 data lines , I can use a lesser amount.

eh.