HART is an important communications protocols used in plant automation. HART is owned by an independent, non-profit HART communication foundation. So that the technology is widely used across the globe as a standard.
HART devices transfer information using HART commands. Each commands hold information, such as measured value of temperature, pressure, flow variables. HART commands are basically in three categories.
Universal command : Command set that all HART devices should support.
Common practice: Common practice command that are recommended. Calibrations commands are included in common practice.
Device or Manufacturer specific command: Command that are specifically done for a propose or the manufacturer.
HART communication system consist of a host system which acts as master, where field devices are the slaves. HART protocol supports two masters, primary and secondary.
Secondary masters are commonly hand held communicators can be used without disturbing master device.
There are basically two types of communication happens between master and the slave.
The host request a specific hard command and the HART device respond with the information. This kind of information is transferred only when the host request.
And there is a Burst communication. The slave continuously broadcast a specific variable continuously.
Multiplexer mode enable a large number of field devices to be connected with the host due to this cascaded multiplexer structure. All devices with address zero.
User connect only a particular current loop for communication via the program. As long as the communication takes place, the multiplexer connect the current loop to the host.
In multidrop mode analog signals are not measured. The devices are only configured to use 4mA as the power for the devices.
Data are transferred only via HART protocol.
Multidrop mode support only 15 devices to connect in parallel. Each devices needed to have distinct addresses range from 1 to 15 .
Control valves cannot include as field device in multidrop mode, because digital HART communication is too slow.