This example is a bit bang implementation. It is not a "serial" transfer using a UART to talk to I2C.

Specifically the SCL line is driven from the RTS handshake signal output from the PC. The SDA signal is driven by the DTR handshake signal coming out from the PC. Reading the SDA line at time of input from the DS1621's in the example is done by monitoring the state of the CTS input hand shake signal.

There is a serious design flaw with this circuit. If one has the RTS and the DTR handshake signals swinging through range of +/- 10V as may be typical for some PC serial ports then when the signal lines are at -10V the input signal levels on SCL and SDA at the DS1621 will be at levels of some negative voltage determined by the clamping action of the zener diodes being forward biased. Typical zeners like this will clamp at voltage of about 0.7 volts. This makes the inputs to the chip be -0.7 volts. If you check the data sheet for the DS1621 you will note that in the critical ratings:

http://datasheets.maxim-ic.com/en/ds/DS1621.pdf

ABSOLUTE MAXIMUM RATINGS
Voltage on Any Pin Relative to Ground -0.5V to +6.0V

This ends up operating the temp sensors outside the absolute limits allowed by the manufacturer. Chip damage is likely to occur over time!!.

Just to qualify my point I looked up a typical 5.1V zener diode at this link:
http://www.diodes.com/datasheets/ds30414.pdf

The zener in the referenced circuit with -10V on the serial port hand shake signal will be biased at right about 2 mA through the 4.7K series resistor. The data sheet shows the VF of the diode to be 0.9V at 10mA and so my estimate at 0.7 volts is totally reasonable.

Another zener diode with 5.1V rating:
http://www.onsemi.com/pub/Colla...1BT1-D.PDF

Specs the VF of the zener at 0.95V at 10mA.

So be very careful about suggesting to deploy circuits of this type that you find on the web. Sometimes, like in this case, the circuits are crap.

Michael Karas