RIA15
HART
®
communication
Endress+Hauser 55
15
HART
®
communication
HART
®
(Highway Addressable Remote Transducer) is a well-established industry standard
worldwide that has been tried and tested in the field and has an installed base of over 14
million devices.
HART
®
is a "smart" technology which enables 4 to 20 mA analog transmission and digital
communication simultaneously on the same two wires. The HART
®
protocol is based on
the Bell 202 Frequency Shift Keying standard (FSK). A high-frequency wave (±0.5 mA) is
superimposed on the low-frequency analog signal (4 to 20 mA). Maximum transmission
distances depend on the network structure and ambient conditions.
In many applications, the HART
®
signal is only used for configuration. However, using the
appropriate tools, HART
®
can be used for device monitoring, device diagnostics and
recording multivariable process information.
The HART
®
communication protocol operates on the principle of master/slave. This means
that each communication activity is initiated by the master during normal operation.
Unlike other master/slave communication types, HART
®
allows two masters in one loop/
network: a primary master, e.g. the control system, and a secondary master, e.g. a
handheld terminal. However, two masters of the same type are not permitted
simultaneously. Secondary master devices can be used without affecting communication to
and from the primary master. The field devices are generally the HART
®
slaves and
respond to HART
®
commands from the master sent directly to them or to other devices.
The HART
®
specification defines that master devices send voltage signals, while the
sensors/actors (slaves) convey their messages using load-independent currents. The
current signals are converted to voltage signals at the internal resistance of the receiver (at
its load).
To ensure reliable signal reception, the HART
®
protocol specifies the total load of the
current loop - including the cable resistance - to be between minimum 230 Ω and
maximum 600 Ω. If the resistance is less than 230 Ω, the digital signal is extremely
damped or short-circuited. HART
®
communication resistance is thus always required in the
4 to 20 mA cable for a low-impedance supply.
15.1
HART
®
protocol command classes
Each command is assigned to one of the following three classes:
• Universal commands
supported by all devices using the HART
®
protocol (e.g. device designation, firmware no.
etc.)
• Standard commands (common practice commands)
Offer functions which are supported by many but not by all HART
®
instruments (e.g.
read out value, parameter configuration etc.)
• Device specific commands
Provide access to device data which are not HART
®
standard but rather unique to a
particular device type (e.g. linearization, extended diagnostic functions)
The HART
®
protocol is an open communication protocol which interfaces the master
device with the field device and can be implemented by any manufacturer and freely
employed by the user. The required technical support is provided by the HART
®
Communication Foundation (HCF).
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