Uses of HART Using HART digital communication, up to four measurements can be transmitted in a single message. Multivariable instruments have been developed to take advantage of this. In addition, when using only digital communication, several instruments can be connected in parallel "multidrop" on a single pair of wires (with their analogue currents each set to a minimum value, usually 4 mA). Each device has its own address, so a host can communicate with each one in turn. HART was developed by Rosemount Inc in the mid-1980s, but has been made completely open, and all rights now belong to the independent HART Communication Foundation, which supports the protocol and oversees further development. There are now upward of 100 member companies in the HCF, most of whom have HART products on the market. Virtually all measurement types are available, also valve positioners. A number of DCSs and PLCs provide HART-compatible inputs, so they can check device status, and read multiple measurements from a single field instrument. You can find out more about available products from the Virtual HART Book. For more details of the technology, try the HCF web site at www.hartcomm.org, or ask your favourite instrument supplier. There is also a technical overview booklet, available from me or from the HCF.