The HART standard also supports a mode of operation that is totally digital, and capable of
supporting multiple HART instruments on the same pair of wires. This is known as multidrop
Every HART instrument has an address number, which is typically set to a value of zero (0). A
network address is a number used to distinguish one device from another on a broadcast network,
so messages broadcast across the network may be directed to specific destinations. When a HART
instrument operates in digital/analog hybrid mode, where it must have its own dedicated wire pair
for communicating the 4-20 mA DC signal between it and an indicator or controller, there is no
need for a digital address. An address becomes necessary only when multiple devices are connected
to the same network wiring, and there arises a need to digitally distinguish one device from another
on the same network.
This is a functionality the designers of HART intended from the beginning, although it is
frequently unused in industry. Multiple HART instruments may be connected directly in parallel
with one another along the same wire pair, and information exchanged between those instruments
and a host system, if the HART address numbers are set to non-zero values.
Setting an instrument’s HART address to a non-zero value is all that is necessary to engage
multidrop mode. The address numbers themselves are irrelevant, as long as they fall within the
range of 1 to 15 and are unique to that network.
The major disadvantage of using HART instruments in multidrop mode is its slow speed.
Due to HART’s slow data rate (1200 bits per second), it may take several seconds to access a
particular instrument’s data on a multidropped network. For some applications such as temperature
measurement, this slow response time may be acceptable. For inherently faster processes such as
liquid flow control, it would not be nearly fast enough to provide up-to-date information for the
control system to act upon.