The HART digital communication signal has a response time of approximately 2‐3 data updates per second without interrupting the analog signal. A minimum loop impedance of 230 Ω is required for communication though 250 Ω is typically used in practice. HART communication occurs in two modes:
(a) Request – Response Mode
(b) Burst Mode
During normal operation (2‐3 data updates per second), each field device (slave) communication is initiated by a host (master) communication device. Two hosts can connect to each HART loop. The primary host is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC). The secondary host can be a handheld terminal or another PC. Field devices include transmitters, actuators and controllers that respond to commands from the primary or secondary host.
Some HART devices support an optional burst communication mode. Burst mode enables faster communication (3‐4 data updates per second). In burst mode, the host instructs the field device to continuously broadcast a standard HART reply message (e.g., the value of the process variable). The host receives the message at the higher rate until it instructs the device to stop bursting. Burst mode is used when more than one HART device is required to listen to communication from the HART loop.
HART Communication Networks
HART devices can operate in one of two network configurations—point‐to‐point or multi-drop.
Point-to-Point Network Mode
In point‐to‐point mode, the 4‐20mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART Protocol. The 4‐20mA analog signal is not affected by the HART signal and can be used for control. The HART communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance and diagnostic purposes. A typical point-to-point communication network is shown below:
The HART Communication Protocol enables several instruments to be connected on the same pair of wires in a multi-drop network configuration. The current through each field device is fixed at a minimum value (typically 4mA) sufficient for device operation. The analog loop current does not change in relation to the process and thus does not reflect the primary variable. Communications in multi-drop mode are entirely digital. Multi-drop connection is mostly used for supervisory control installations that are widely spaced such as pipelines, custody transfer stations, and tank farms. Below is shown a multi-drop network configuration: