Out in Front: December 7, 1995

The DS1820 digital thermometer from Dallas Semiconductor solves a problem of multipoint temperature sensing—the problem of usually requiring lots of parallel wiring or an add-on multidrop interface with each sensor. The DS1820, a direct-to-digital sensor, incorporates circuitry that lets you multidrop many devices on a twisted-pair cable up to 300m long. A laser-burned ROM within each sensor provides a unique serial number, enabling the host processor to identify each sensor. The precise number of drops is a function of cable loading, not available sensor codes; typically, the multidrop supports hundreds of points. The sensors measure –55 to +125° in 0.5° increments and send a 9-bit reading to the host. Applications include building HVAC and process monitoring.

The sensor requires no external components and can receive power from the data line itself using energy stored in an internal parasitic capacitor. This capacitor recharges when the signal line is high; in this situation, standby operating power is zero. The sensor can also receive power from a local 5V supply, recommended in some applications and for reliable operation at temperatures higher than 100°.  A master/slave protocol in each sensor defines communications between the host and sensors. Each sensor also includes user-settable nonvolatile high- and low-alarm levels. The host processor uses an alarm-search command to identify and address those sensors with temperatures exceeding the programmed limits. The DS1820 is available as a three-terminal device or in a 16-lead SSOP and costs $2.77 (10,000). -- by Bill Schweber

Dallas Semiconductor Corp, Dallas, TX. (214) 450-0448.