Newsgroups: sci.electronics.misc
Subject: Re: easiest way to "fake" an rs232 signal ?
References: 
P Oster  writes:

> Hi,
> 
> I'd like to set up a device that triggers on a voltage threshold and
> sends a signal to a PC over the RS232.  The data doesn't matter, just
> the fact that _something got sent over the Tx line and it's time would
> be known within a few fractions of a second.
> 
> I don't want to learn to program a micro, what's the simplest circuit
> that can "fake" an RS232 signal ?

You need a circuit that will give out a pulse that has suitable 
length. Basically a positive pulse with length between one and 9 bit 
times (bit time determined by the communications speed you plan to use) 
sent to RS-232 port would be recongized as some character sent to 
the serial port (or in unfortunate case character received and 
and also marked that this was noisy character if pulse ends jut in the 
middle of the pulse time). 

This kind of pulse can be generated with 555 timer, any simple 
monostable multivibrator. I once made some experiments to 
detect some pulse starting by using just one capacitor + 
resistor and high pass filter between pulse source and RS-232 
port... after some thweaking I got the the signal from 
he external device signal sharp rising edge to something 
that computer recognizes as one character beign recevied.

Or using the sallest microcontroller 
you can find (some small PIC in 8 pin case with internal 
oscillator would be pretty compact in component count, and 
could even send real meaningful RS-232 data).      
 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/