Dispelling the myths of HART-enabled devices

 By Ed T. Ladd, Jr., HCF

From its inception, the power of the HART Protocol has been underutilized due to misconceptions regarding the benefits of the technology. A primary misconception that &ldquoHART is only good for device setup, calibration and maintenance&rdquo was cemented in users minds because that&rsquos how they initially used HART&mdashwith handheld communicators, the only HART host technology available at the time.

So even though handheld communicators and their ability to reprogram and diagnose HART devices has positioned HART as the most used field communications protocol in the industry today, they have also conditioned users to believe that this is the only viable use for HART Communication. And, that misconception costs users of HART-enabled devices dearly everyday.

Field communications is one of the fastest growing technology areas within our industry, perhaps the only area experiencing growth. From Ethernet to wireless, protocols tout communication speed specifications as their number one advantage over HART, propagating the myth that &ldquoHART Communication is too slow for use in control systems.&rdquo This myth is quickly dispelled when we evaluate how the HART Communication technology works and all that it can do.

HART-enabled device&rsquos provide two simultaneous communication channels transmitted via a single pair of wires. This technological approach offers the fastest possible data transfer of a single Primary Control Variable via the analog channel, which is backward-compatible with most existing analog I/O systems and faster than any other field communications protocol.  Response from the 4-20mA analog signal is almost instantaneous depending upon the signal dampening built into the system. 

A HART device&rsquos second communication channel, the digital channel, transmits intelligent device information on the same pair of wires as the Primary Control Variable. The digital channel provides information on the physical device (i.e. tag, manufacture, model), device status (i.e. sensor failure, reset condition, configuration change), calibration (i.e. upper range value, lower range value, dampening) and additional device variables.

When using HART Communication&rsquos digital channel, system I/O design is critical to the performance of a HART field network. HART field networks can be configured in a point–to-point or a multi-drop topology, but most installation utilize the point-to-point topology. A point-to-point topology takes advantage of the two simultaneous communication channels and eliminates any data speed reductions caused by &ldquodaisy-chaining&rdquo multiple devices on a single loop. A point-to-point HART field network configured with a 1:1 modem ratio or one modem per device, responds with two to three updates per second.  In other words, it takes approximately 380 milliseconds to respond to a request for information. 

In many cases may not be required, but to show how I/O design can affect performance, the same network configured using a multiplexer with an 8:1 modem ratio (8 devices to 1 modem) actually cuts the data throughput in half. While designing a HART network using a remote I/O with a 1:1 (one modem: one device) modem ratio, you can achieve results as if there were only one device connected to the network. Table 1 provides statistical information on system latency, the time that is required for the system to receive a response based on the HART I/O used. It is important to note that not all HART I/O designs are equal. 

Table 1 - HART I/O Latency (in seconds)

Number Channels	Point-to-Point (1:1 ratio)	Multi-Drop (Ch#:1 ratio)	Multiplexed (8:1 ratio)
1	0.38	0.38	0.38
4	0.38	1.51	2.80
8	0.38	3.02	5.60
16	0.38	6.04	11.21
32	0.38	12.08	22.43
256	0.38	---	179.42

As was discussed earlier, the intelligent information doesn&rsquot stop with the analog Primary Control Variable signal. In addition to analog PV transmission, real-time status, diagnostics and multi-variable information are communicated via the digital channel on the same pair of wires with the analog signal.