HART digital communications uses a request/reply communications model. This means that in general, HART devices won’t transmit any information unless a request is sent from the host to the device. The exception to this is the burst mode where the HART device can send a single piece of information continuously without repeated host requests. A common use for burst mode is to send the process variable as a digital value to verify the analogue signal. Many control systems aren’t designed to accept HART information in digital form so it is common to see external multiplexers reading the digital signal. In this approach, the HART device is attached to both the control host and to the multiplexer.
Although this increases the cost of the installation, reductions in maintenance cost generally pay back the investment in a very short time. Some hosts, such as Emerson’s DeltaV are able to capture and pass HART digital information to other applications (e.g., AMS Asset Management Suite) using a mechanism commonly called “HART pass through”. Using a system that supports HART pass through reduces the cost of acquiring and using the HART information by eliminating the need to install separate multiplexer systems.