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We can use the control signals from a serial port similarly (reset=rts sck=dtr mosi=txd miso=cts). The serial port outputs +-12V, so we need a few zener diodes to transform this to 0V/5V:
Schematic: 
My version:
(bigger images attached)
AVRDude calls this design dasa. It also supports dasa2 and dasa3, which are similar, but with slightly different pinouts. dasa2 is reset=!txd sck=rts mosi=dtr miso=cts. dasa3 is reset=!dtr sck=rts mosi=txd miso=cts. I am including the design for dasa, since it is the only one I have used. The claim is that the dasa3 design may have better performance on some serial ports, since DTR was not designed to toggle quickly.
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