>This application is nearly what I was looking for, but I cannot
>see, how the needed *negative* voltage for the Tx line is generated.

It isn't. Most of these schemes rely on the genuine RS232 port having a 
threshold of around 1.5V (typical for an 1489 receiver chip, and many 
similar chips) and hence do not drive the line negative. What you loose is 
noise immunity on the negative side, but most of the time this does not 
matter, or a protocol is designed to operate in a noisy environment.

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