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Description: The intelligent temperature sensor DS18S20 from Dallas Semiconductor is a calibrated, precise two- or three-pin device that interfaces directly with microcontrollers or PCs to produce Celsius readings. The so-called 1-wire protocol (better referred to as "one DATA wire") can be adapted to the RS232 port with minimal component count. Simple circuits have been published that do not work well with new DS18S20 components, but improvements are underway. Modern desktop or laptop computers no longer provide serial RS232 interfaces; instead, they feature numerous USB interfaces. USB is an intelligent interface that requires an intelligent IC to signal the type of interface to the computer upon connection. Additionally, more power, up to 2 W, is available to operate the 1-wire bus circuit. The FTDI USB-to-serial interface chip provides the electrical interface to the computer and simulates a "fake" serial protocol. When properly applied with suitable hardware, the computer may still believe it is operating a serial interface while in fact using USB. A prototype board of such an interface is shown, utilizing a USB-to-serial chip to provide "Transmit TXD" and "Receive RCV" signals that ultimately operate a 1-wire hardware interface. The circuit employs a ready-made kit interface adapter board for the FTDI USB-RS232 chip in surface mount technology. The 1-wire circuit interface includes a voltage doubler circuit to ensure sufficient drive voltage and current for the 1-wire bus, even when the USB voltage is lower than normal. A zener diode at the output node limits the drive voltage and protects against external overvoltage or cable noise. Compared to a passive pull-up resistor, this circuit provides substantial loop current even at voltages just below the high-level voltage. Some of the 74HC14 gates are utilized to detect and invert the 1-wire bus signals for the serial interface. A simple timer circuit is implemented to make the operation LED flicker during 1-wire bus operation. When using the Digitemp software in Linux, the software functionality is identical to the traditional passive interface but improved for higher temperature applications of the DS18S20 device. Using long wires through workshops or buildings connected to the computer's ground line poses significant risks concerning EMI and noise impact, as well as operator safety. A system with isolated grounds, where the 1-wire bus is separated from the USB and the computer, enhances the reliability and availability of the measuring system, even under the influence of electrical noise, lightning, or ground voltage disturbances. The isolated transfer of power utilizes a transformer and rectifier assembly, constructed with a digital bus driver circuit (74HC245) and a 1:1:1 ferrite transformer (three identical windings). This design offers low power consumption, high efficiency, and the use of readily available low-cost parts. The transfer of RS232 information employs high-speed optocouplers (6N136). A low-cost hex Schmitt inverter HCMOS IC (74HC14) is used on the isolated side for processing filtered 1-wire signals and driving the LED, thereby reducing the overall component count. Highly accurate reference frequencies can be generated in relation to radio broadcast frequencies controlled by national standard institutes (e.g., Germany's DCF77 at 77.5 kHz, UK's Rugby at 60 kHz). At these long wave frequencies, receivers typically use a directional ferrite rod antenna along with a straightforward amplifier and detector. Good reception in a noisy environment is crucial to minimize frequency jitter associated with poor receiver quality. When filtered with a 77.5 kHz quartz filter, the resulting bandwidth may be too narrow to accurately detect modulated coded seconds and time marks.The intelligent temperature sensor DS18S20 from Dallas semiconductor is a calibrated precise two resp. three pin device that interfaces direchtly to microcontrollers or PCs producing Celsius readings. The so-called 1-wire protocoll (better "one DATA wire. ) can be adapted to the RS232 port with minimal component count. Simple circuits are publishe d that do not work too well with new DS18S20 components. But improvements are on the way. Latest desktop or laptop computers do not provide serial RS 232 interfaces any more, but instead numerous USB interfaces. USB is an intelligent interface, in every application it requires an intelligent IC to signal the type of interface to the computer when USB gets connected.

On the other side more power, up to 2 W, is available to operate the 1-wire bus circuit. The FTDI USB-to-serial interface chip provides the electrical interface to the computer as well as the functions of a "fake" serial protocol. When properly applied with suitable hardware the computer may still believe to operate a serial interface when in fact USB is used.

Below a prototype board of such an interface is shown that uses a USB-to-serial chip to provide the "Transmit TXD" and "Receive RCV" signals that finally operate a 1-wire- hardware interface. The circuit uses an ready-made kit interface adapter board for the FTDI USB-RS232 chip in surface mount technology.

The 1-wire circuit interface includes a voltage doubler circuit to provide "always" enough drive voltage and current for the 1-wire bus even in case when the USB voltage is lower than normal. The zener diode at the output node limits the drive voltage as well as external overvoltage or cable noise.

Compared to a passive pullup resistor this circuit provides substantial loop current even at voltages just below the high level voltage. Some of the 74HC14 gates are used to detect and invert the 1-wire bus signals for the serial interface.

A simple timer circuit is used to make the operation LED flicker in case of 1-wire bus operation. When using the digitemp software in Linux the software functionality is identical to the traditional passive interface, but improved with respect to higher temperature applications of the 18S20 device. more. . (2011) Using long wires through workshops or buildings connected to the precious computer`s ground line is not the greatest idea concerning EMI and noise impact as well as operators` safety.

A system with separated grounds, the 1-wire-bus isolated from the USB and the computer, would improve reliability and availability of that measuring system even under influence of electrical noise, lightning or ground voltage disturbances. The isolated transfer of power uses a transformer and rectifier assembly, here build with a digital bus driver circuit 74HC245 and a 1:1:1 ferrite transformer (3 identical windings).

This provides low power, high efficiency and utilisation of readily-available low-cost parts. The transfer of RS232 information uses high speed optocouplers 6N136. A low-cost hex Schmitt inverter HCMOS IC 74HC14 is used on the isolated side for the processing of the filtered 1-Wire-signals and LED drive to reduce the overall component count. Highly accurate reference frequencies can be generated in reference to radio broadcast frequencies that are controlled by national standard institutes (Ger DCF77 77k5, UK Rugby 60k etc.

) At these long wave frequencies receivers usually employ a directional ferrite rod antenna and a straightforward amplifier and detector. Good reception in a noisy environment is necessary to reduce the frequency jitter that is seen with too poor receiver qualities.

If filtered with a 77k5 quarz filter the resulting bandwidth is too small to detect the modulated coded seconds` and time marks.