wrote:

>So I'd like to get some advise about wiring a transistor switch to switch
>the -12V available on DTR instead of passing 0V. So that 5V will pass 5V but
>0V will turn on the transistor and pass -12V.
>

Derivation of several published circuits:

http://www.oricomtech.com/appnotes.htm
AN-ECD02 - A Simple Inexpensive Network-Compatible RS-232 Interface:
an-ecd02.zip

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