The HART protocol communication is half-duplex. The host can communicate with a field transmitter and the field transmitter can communicate back to the host. However, there is a single communication line across the 4-20mA loop, and only one or the other can send commands or data at any given time.

There are two modes of HART communication between a host and a field device. The first mode of communication is a request-response mode. The host sends a command and the field transmitter responds. Communications are initiated by the host device.

Second, some HART devices support burst mode communication. In burst mode, the host sends a command to initiate a burst mode response. Once started, the field transmitter responds with a repeated HART response. For example, the response can be a digital readout of the primary variable. The field transmitter repeatedly sends out the value of the primary variable, until the host sends a command to stop burst mode. Figure 1-7 and Figure 1-8 show the frame communication for HART devices in request-response and in burst mode. The transmitted primary variable can be in units for a variety of measurements used for process control.

With long HART packets and a baud rate of 1200 bps, request-response mode communications are as fast as two to three data updates per second. Burst mode increases the data update to three to four updates per second.