Use directions in Dallas chip directly and properly, instead of isolation and get that working first. Then show waveforms if flawed.
To provide energy for a temperature conversion or for copying data from the scratchpad to the EEPROM,a low-impedance pullup of the 1-Wire bus to 5V is required just after the corresponding command has been sent by the master. During temperature conversion or copying the scratchpad, the bus master controls the transition from a state where the data line is idling high via the pullup resistor to a state where the data line is actively driven to 5V, providing a minimum of 1mA of current for each DS1920 doing temperature conversion. This low impedance pullup should be active for 0.75 seconds for temperature conversion or at least 10ms for copying to the scratchpad. After that, the data line returns to an idle high state controlled by the pullup resistor.
read the following
in particular ... impedance matching and active pullup
Consider for EMI reduction, and use twisted pair with CM choke for improved noise rejection.
If you need isolation for hiV or CM rejection give reasons and consider above instead.
Assuming your previous thermal questions are related, do you have the negative side of the Thermocouple wire grounded to aluminum block? The tip must be electrically insulated yet thermally coupled, perhaps with oil or thermal grease.
Thermal oils are available in a wide range of specifications. Different manufacturers produce different oils under different brand names. Different specifications suit various needs of the process and applications. Some of the commonly used brands are
- Therminol from Solutia Inc.
- Dowtherm from Dow Chemical Co.
- Exceltherm from Radco Industries Inc.
- Paratherm from Paratherm Corp.
The most important characteristic is the maximum temperature of service. The oil is thermally stable till this temperature. It should be borne in mind that the cost increases exponentially with increase in the maximum temperature limit. Currently the thermal oils are available up to 400 °C.