Brochure/Catalogue
Glossary
HART
®
HART
®
(Highway Addressable Remote Transducer) is a communications protocol
for bus-addressed eld devices used in process automation. In HART
®
-based
communications, eld devices and controllers are connected together over 4–20
mA current loops. This analogue signal is superimposed with a digital signal by
using the FSK process (Frequency Shift Keying). The process allows additional
measurements, conguration and device data to be transmitted without
inuencing the analogue signal. Ex isolators can also be used in hazardous areas.
Time
4 mA
20 mA
Digital
signal
Analogue
signal
Bit 1 Bit 0 Bit 0 Bit 1 Bit 1 Bit 0 Bit 1
Hazardous area According to the ATEX directive, an hazardous area is where the extent of the
explosive atmosphere mandates that extra measures must be taken to safeguard
health and protect surrounding machinery. Hazardous areas are classied
according to the frequency and duration of the occurrence of the explosive
atmosphere (refer to the sub-divided zones).
Hysteresis Species the percent difference between the switch-on and switch-off points
of a switching contact. The hysteresis must not fall below a minimal value.
Otherwise it would no longer be possible to carry out specic switching during
the monitoring of threshold.
I
IECEx An international directive regarding the creation of declarations of conformity by
the manufacturers of facilities, devices and components that are intended for use
in explosion risk zones. This directive is valid throughout the globe but is only
currently used in some Asian nations.
Impulse withstand voltage The high pulse voltage of a specied form and polarity that does not lead to an
insulation breakthrough or ashover, under the specic conditions dened in
EN 60664-1.
W
Technical appendix/Glossary
W.292729490000