com DS9097 (passive)
DS9097 Passive Serial Adapter
Notes: The DS9097 is the original serial 1-wire adapter. It is easy to construct from passive elements.
The "E" version used a DB25 rather than DB9 serial connector and had an optional 5V external power source for powering a bus line.
There are several published designs for making the adapter:
Dr Harald Roele (including schematics and Eagle layout)
Prof. Dr.-Ing. Manfred Hamann (including powered and unpowered)
Marcel Thranhardt's powered circuit.
Embedded Data System's HA2 (Not advertised as a DS9097 clone, but see this thread).