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FAQTEMP08 FAQ
Answers to the questions TEMP08 owners asked most often
Most TEMP08 problems came down to a short list: how to power and wire the sensors, how far the bus could run, which serial settings to use, which temperature chip was which, and why a reading suddenly turned into question marks. The answers below gather that experience in one place, grouped by topic rather than by the order the questions arrived.
First published August 22, 2003 · Text rewritten 2026
Power, parasite mode and supplies
What parasite power means
Parasite power is Dallas Semiconductor's name for running a 1-Wire device from its data line. The bus really uses two conductors, data and ground, and there is no separate supply wire. Whenever the data line sits at logic 1, every device on the bus charges an internal capacitor from it, and that stored charge carries the chip through the moments when the line is pulled low. Because the idle state of a 1-Wire bus is logic 1, the line spends most of its time high and the devices stay topped up.
Parasite mode has conditions. Each device's datasheet says how its supply pin must be treated. The DS18S20, for example, needs its VDD pin tied to ground (VSS) when it runs parasitically. In practice that means connecting the ground wire to both outer legs of the TO-92 package; leaving VDD floating gives unreliable readings. Temperature conversions draw the most current, which is why long parasite-powered runs with many sensors are more fragile than locally powered ones. The TEMP08 offered +5 V on its terminal strip and RJ-12 jack for sensors that prefer a real supply.
Choosing a power adapter for the board
The board itself accepted 12 to 16 V on its J1 input, AC or DC, and the DC input was polarity compensated. That made a spare wall adapter from an old appliance a sensible choice, as long as it could supply at least 100 mA. The board could also run from USB, although the +12 V output on the terminal strip was then not available.
Wiring sensors around a house
Sensors connect to the TEMP08 through twisted-pair cable, normally CAT-5 or better, landing on either the J3 terminal strip or the J5 RJ-12 jack. There are two basic layouts. In a home run, every sensor has its own cable back to the board and all the cable ends are joined at the TEMP08. In a daisy chain, one or more cables leave the board and each sensor is tapped into the run where it is needed.
Daisy-chaining is fine: sensors can join the cable anywhere along its length, and the far end needs no terminator. Just make sure the open end cannot short. With good-quality CAT-5, sensors can be placed up to about 300 feet (100 meters) from the board.
The usual way to attach a sensor is to solder it straight onto the wire run. People who dislike soldering have crimped sensors into four-contact modular plugs and installed matching wall sockets where readings are needed, and the company made small circuit boards that held a DS18S20 with either a modular jack or a two-screw terminal. The pin assignments for those jacks are covered on the RJ11 versus RJ12 pinout page.
Why a star layout can fail
If a sensor is added on a new branch in a star arrangement, the extra branch can create signal reflections, and readings start coming back as ???. The documented fix is a 100-ohm resistor in series with the data (DQ) conductor of each branch. The resistor must sit at the TEMP08 terminal, not further down the cable. For broader layout advice see the 1-Wire network wiring guide.
The DS18 temperature sensor family
Four closely related Dallas/Maxim thermometers appear in TEMP08 questions, and the TEMP08 read all of them, along with the DS1920 iButton. They share the 1-Wire protocol and the −55 to +125 °C range, but differ in resolution and accuracy. The figures below are standard datasheet values; accuracy is quoted for the −10 to +85 °C band.
| Part | Family code | Resolution | Accuracy | Package and status |
|---|---|---|---|---|
| DS1820 | 10h | 9 bit | ±0.5 °C | PR-35 package; production ended around May 2000 |
| DS18S20 | 10h | 9 bit | ±0.5 °C | TO-92; drop-in replacement for the DS1820 |
| DS18B20 | 28h | 9 to 12 bit, selectable | ±0.5 °C | TO-92 and others; common in waterproof probes |
| DS1822 | 22h | 9 to 12 bit, selectable | ±2 °C | TO-92; lower-accuracy economy part |
The DS18S20 was introduced to replace the DS1820. It keeps the same family code and output format, has better accuracy in practice and tolerates a wider supply range. The DS1820 also had a known flaw that produced a reading error of about 1 °C, one of the reasons Dallas brought out the S version.
Parts marked DS1820 that are really DS18S20
Dallas/Maxim did not print the S on the DS18S20. Once the original DS1820 package was discontinued, the maker seems to have assumed nobody would be confused. The visible difference is the package: the old DS1820 came in a PR-35 package, while the DS18S20 uses a standard TO-92. A TO-92 part marked DS1820 is a DS18S20.
Serial link, software and setup
Connecting to a PC
The TEMP08 connects to a PC serial port through a straight-through cable, not a null-modem cable. The link runs at 9600 baud, 8 data bits, no parity and 1 stop bit, with flow control set to none. That last setting matters: with hardware flow control enabled, a terminal program shows nothing. HyperTerminal, or any other terminal emulator, can be used to watch the output and send configuration commands. On machines without a serial port, a USB-to-serial adapter works; the Belkin F5U103 was tested, and others behave the same way.
Programs that can read the TEMP08
HomeSeer is not required. Any application that reads a serial port can collect the readings, because each one arrives as a plain text line. HomeSeer did make the job easy through scripts and plug-ins, and the procedure is written up on the HomeSeer setup page. The freeware thermd program is a stand-alone alternative that logs and graphs the data.
Adding a sensor and telling sensors apart
A newly connected sensor is found with the INI command, which searches the bus and writes any new serial number to EEPROM, normally in the next free position at the end of the list. There is no need to run ERA first; that erases every stored sensor. To remove only some entries, use DEL.
The reliable way to map sensors to rooms is to add them one at a time: connect a sensor, run INI, then note the newest serial number shown by DIS. With serial-number display switched on, that ID appears in every reading. Once a sensor is registered, the TEMP08 reports in real time when it is disconnected or reconnected. All of these commands are described in the TEMP08 command reference.
A three-room starter setup
Reading three rooms needs one TEMP08, a 12 V adapter, a straight-through serial cable and sensors for the rooms. Standard boards carried their own temperature sensor, so if one of the rooms is where the board sits, only two external sensors are needed.
Weather stations, humidity and accuracy
Weather station versions
A weather station is optional; the TEMP08 works with any mix of temperature, humidity, counter and voltage sensors. When one is used, the version matters. Version 1 was the original Dallas Semiconductor unit. AAG, a Mexican company, sold an electrically equivalent Version 2, then redesigned the wind direction sensor to use a DS2450 in place of the older DS2401-based arrangement and sold that as Version 3. The TEMP08 supports Version 3 only, not Versions 1 or 2. Dallas stopped making the station and AAG later appeared to go out of business; Texas Weather Instruments offered a similar product. The 1-Wire weather station guide covers the hardware in more detail.
Humidity sensors
The TEMP08 reads humidity from multiple DS2438-based sensors on the same bus. Dallas/Maxim's own humidity module was discontinued, so most were third-party or kit boards. The Sensirion SHT11 and SHT71 were not supported: their two-wire interface was judged unsuitable for long remote runs of the kind 1-Wire handles well.
Sensors that disagree
Several sensors side by side rarely show identical numbers. A DS18S20 is specified to ±0.5 °C (±0.9 °F), so two parts at opposite ends of that tolerance can differ by up to 1.8 °F while both are in spec. In 2004 Dallas/Maxim also had a production problem with lots carrying batch code B7, where factory calibration could be badly wrong. Old DS1820 parts add their own offset of about 1 °C.
Outdoor sensors that read high
Direct sunlight heats a sensor above air temperature. The fix is a ventilated radiation shield, often called a pagoda, which keeps sun and rain off while letting air pass.
Water, pools and tanks
Sensors can go into water, provided the sensor body, leads and joints are sealed with silicone RTV or epoxy. Ready-made waterproof DS18B20 probes save the effort.
Switch chips and enclosures
Later firmware no longer read the DS2405, DS2406 and DS2407 switch chips; the company's 1WSwitch interface handled those. The board fits a standard PacTec CM5-125 enclosure.
Further reading: DS18B20 product page, Analog Devices · DS18S20 product page, Analog Devices · DS1822 product page, Analog Devices
Questions and answers
How far can TEMP08 sensors be from the board?
With good-quality CAT-5 twisted-pair cable, sensors can be placed up to about 300 feet (100 meters) from the TEMP08. Long star layouts with many branches are more prone to reflections than a single daisy chain, so keep the layout simple and add series resistors on branches if readings become unreliable.
Why do my readings show ??? after adding a sensor?
The new sensor was probably added on a fresh branch of a star layout, which causes reflections on the bus. Fit a 100-ohm resistor in series with the data (DQ) conductor of each branch, placed at the TEMP08 terminal rather than out along the cable, and the readings should return.
Is a DS1820 the same as a DS18S20?
Not quite. Both use family code 10h and the same output format, but the DS18S20 has better accuracy and wider supply tolerance, and the DS1820 had a flaw causing a 1 °C error. DS18S20 parts were printed DS1820 without the S; a TO-92 package means DS18S20, the older PR-35 package means DS1820.
Can the TEMP08 read DS18B20 and DS1822 sensors?
Yes. The TEMP08 reads the DS18S20, DS1820, DS18B20, DS1822 and the DS1920 iButton, and they can all share one bus. The DS18B20 offers 9 to 12 bit resolution at ±0.5 °C, while the DS1822 has the same resolution options but a looser ±2 °C accuracy.
What serial settings does the TEMP08 use?
9600 baud, 8 data bits, no parity, 1 stop bit, and flow control set to none. The cable must be straight-through, not null-modem. Leaving hardware flow control enabled in the terminal program is the most common reason for seeing no output at all.
Which weather station works with the TEMP08?
Only the Version 3 1-Wire weather station from AAG, which uses a DS2450 for wind direction. The original Dallas Semiconductor Version 1 and the AAG Version 2 are not supported. A TEMP08 bus can carry one weather station alongside its other sensors.
Related pages
TEMP08 Serial One Wire Interface
TEMP08 linked a PC serial port to the 1-Wire bus: up to 60 sensors for temperature, humidity, rain, wind and relays, plus a real-time clock and RJ-12 jack.
TEMP08 Instructions
TEMP08 command reference for firmware 2.24 and later: DEL, DIS, INI, SET, SPT and more, with SET options, DIS output codes and how to read a TMP poll line.
RJ11 versus RJ12 connectors
RJ11 vs RJ12 explained for 1-Wire: 4-contact and 6-contact modular jacks, pin numbering, and where DQ, ground and power sit on TEMP08 and Maxim wiring.
TEMP05 Frequently Asked Questions
Answers for TEMP05 owners: parasitic power, sensor wiring and range, identifying sensors, firmware support, weather station versions and RJ11/RJ12 pinouts.
1-Wire Humidity Sensor
How a 1-Wire humidity sensor works: the DS2438 circuit with an HIH-4000 element, temperature compensation, realistic accuracy limits and good placement.