Uses of HART　　　　　　Using HART digital communication, up to four measurements can be tra mitted in a single me age. Multivariable i truments have been developed to take advantage of this. In addition, when using only digital communication, several i truments can be co ected 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 addre , 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 su orts 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 DC  and PLCs provide HART-compatible i uts, so they can check device status, and read multiple measurements from a single field i trument.　　　　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 i trument su lier. There is also a technical overview booklet, available from me or from the HCF.