Can Intelligent Transmitters Communicate Digitally with DCS Systems?

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Why do some companies offer multiple series of intelligent transmitters?

Intelligent transmitters have high precision. However, in production process detection and control, there are times when extremely high instrument precision isn't necessary—only stable performance and relatively accurate measurements are required.

For this reason, while developing expensive high-performance intelligent transmitters, many companies also produce economical intelligent transmitters with lower performance and lower prices. Examples include Honeywell's ST3000/100 (high-performance) and ST3000/900 (economical), Rosemount's 3051C and 1151S, and Fuji's FCX-A/AX (high-performance) and FCX-C (economical). In systems where economical transmitters meet requirements, selecting them can significantly reduce investment costs.

Can intelligent transmitters communicate digitally with DCS systems?

Whether an intelligent transmitter can communicate digitally with a DCS depends on specific circumstances:

① If the intelligent transmitter and DCS use the same communication protocol, digital communication is theoretically possible, but in practice, additional agreements are often required—otherwise, communication will fail. Even between a company's own intelligent transmitters and DCS, for example: Honeywell's TDC-3000 DCS (using DE protocol) and ST3000 intelligent transmitters can communicate only if the TDC-3000 is equipped with an intelligent card (STI); if it has a 4~20mA analog card (high-level input card), only one-way 4~20mA analog signal transmission is possible, not digital communication. Similarly, Yokogawa's CENTUM XL DCS (supporting HART protocol) can only work with its own EJA intelligent transmitters, and even then, a digital detection card (ESC) must be installed in the system—otherwise, only 4~20mA analog values can be transmitted.

② While it's reported that systems and intelligent transmitters from different companies can communicate digitally if they share the same protocol, this is rarely achievable in practice.

Both Fuji and Rosemount intelligent transmitters comply with the HART protocol, so why can Rosemount's handheld terminals program and configure Fuji transmitters, but Fuji's handheld terminals cannot do the same for Rosemount transmitters?

The HART communication protocol was primarily developed by Rosemount. When instrument companies first developed intelligent transmitters, HART had not yet become a unified standard, so each company developed products according to its own communication protocols. After HART became a unified industrial standard, many instrument companies only adapted to it through conversion methods. Thus, Rosemount's 275 handheld communicator can operate Fuji's FCX-A/C intelligent transmitters—but to correctly identify Fuji's product models, the 275 must be loaded with special support files developed by Fuji. This applies to Rosemount's operation of other HART-compliant transmitters as well. As for other companies' handheld terminals (e.g., Fuji's FXW, Yokogawa's BT200), they cannot operate Rosemount's 3051C intelligent transmitters, nor can they communicate with each other or other HART-supported transmitters.

When a transmitter's actual operating range is not its maximum range, can its accuracy still be guaranteed?

New transmitters allow range setting according to usage needs. This range can be the maximum range or a smaller one, but it cannot be too small—beyond a certain point, accuracy will degrade.

For a 0.065-class differential pressure transmitter, the relationship between accuracy and operating range is generally expressed by the following formula:

For a differential pressure transmitter with a maximum range of 100kPa, x = 10kPa; for one with a maximum range of 0~10kPa, x = 3kPa.

The International Organization for Standardization (ISO) defines a new term: "rangeability," which is the ratio of the "maximum upper range value" to the "minimum upper range value."

Manufacturers guarantee that for a differential pressure transmitter with a maximum range of 100kPa, the upper rangeability is 10; if the range is below 10kPa, accuracy will drop below 0.065%. For a transmitter with a maximum range of 10kPa, the upper rangeability is 3.3; if the range is below 3kPa, accuracy will fall below 0.065%.

True or false: Since the zero point (including positive/negative zero migration) and range of intelligent transmitters can be set and modified via a handheld communicator, there's no need to calibrate them using pressure signals.

False. While it's true that the zero point (including positive/negative zero migration) and range of intelligent transmitters can be set/modified via a handheld communicator—allowing operators to remotely adjust the measurement range without being on-site, which is beneficial for meeting production needs promptly, reducing labor intensity, and especially useful in toxic or high-altitude areas where access is difficult—the correctness of remote settings cannot be verified or adjusted through the handheld communicator alone. Only by applying actual pressure and comparing it with the instrument's indication can accurate zero points and measurement ranges be achieved. Therefore, intelligent transmitters still require calibration using pressure.

However, because intelligent transmitters are based on microprocessors with self-diagnostic functions, even without applying actual pressure, setting deviations are minimal. Typically, if an intelligent transmitter was originally qualified, it should remain qualified after zero and range adjustments via a handheld communicator—any error exceeding specifications will be small, and given the high precision of these transmitters, minor deviations won't affect usage. But if a transmitter was originally unqualified, adjusting its range won't make it qualified; calibration is necessary before use.

When were intelligent transmitters introduced, and what are their characteristics?

In the early 1980s, Honeywell (USA) first launched the ST3000 series of intelligent pressure transmitters—a natural product of advancements in computer and communication technologies. Soon after, other global instrument companies introduced similar intelligent transmitters. These instruments share the following characteristics:

① In addition to pressure (differential pressure) sensing elements, their detection components typically include temperature sensing elements. Using micro-electro-mechanical system (MEMS) processing, ultra-large-scale application-specific integrated circuits , and surface-mount technology, these instruments feature compact structures, high reliability, and small sizes.

② Intelligent transmitters offer high accuracy (generally ±0.1% to ±0.2%, some even reaching ±0.075%), wide measurement ranges (turndown ratios of 40:1, 50:1, 100:1, or even 400:1), and significant improvements in temperature performance, static pressure performance, and one-way overload capacity compared to previous transmitters.

③ The zero point and range of intelligent transmitters can be remotely set via a handheld communicator (also called a hand operator or handheld terminal), allowing range adjustments without applying signal pressure—particularly convenient for inaccessible locations.

④ Intelligent transmitters can achieve digital communication with DCS control systems, laying the groundwork for fully digital fieldbus control systems.