How can you implement a HART communication protocol in a DCS system?

1 What is a DCS system?

A DCS system is a network of controllers, sensors, actuators, and other devices that work together to monitor and control a complex industrial process. A DCS system can have multiple levels of hierarchy, such as field, control, supervisory, and management, and can communicate with different protocols, such as Ethernet, Modbus, Profibus, or HART. A DCS system can provide centralized or distributed control, data acquisition, analysis, visualization, and optimization of the process.

2 How does HART work?

HART is a hybrid protocol that combines analog and digital signals. It uses the 4-20 mA current loop as the analog carrier, and superimposes a low-power, high-frequency digital signal on top of it. The analog signal represents the primary process variable, such as pressure, temperature, or flow, while the digital signal carries additional information, such as device status, diagnostics, configuration, or calibration. HART can operate in two modes: point-to-point or multidrop. In point-to-point mode, one master device (such as a controller or a handheld communicator) can communicate with one slave device (such as a transmitter or a valve) over a pair of wires. In multidrop mode, multiple slave devices can share the same pair of wires, but only the digital signal is used.

3 How to connect HART devices to a DCS system?

There are various ways to connect HART devices to a DCS system, depending on the existing infrastructure and the desired functionality. For instance, some DCS systems have HART-compatible I/O modules that can read both the analog and digital signals from the HART devices and send HART commands to them. This method allows full access to the HART data and functionality, but may require additional hardware or software upgrades. Alternatively, a HART multiplexer can connect multiple HART devices to a single communication channel, such as a serial port or an Ethernet port. The multiplexer can act as a master device and communicate with the HART devices using the HART protocol, then convert the data to another protocol, such as Modbus or OPC, that can be read by the DCS system. However, this method only allows access to the HART data, not to its functionality. Finally, a HART converter can convert the digital signal from a HART device to an analog signal that can be read by a standard I/O module. This method allows access to some of the HART data but not to its functionality.

5 What are the challenges of using HART in a DCS system?

Using HART in a DCS system can also present some challenges, such as limited bandwidth of 1.2 kbps, which can affect the response time and data resolution of HART communication, as well as potential interference from other sources, such as power lines or motors. Additionally, compatibility issues with some devices or systems, especially older or non-standard ones, can result in data loss or misinterpretation. Consequently, it is essential to verify the compatibility and interoperability of the HART devices and systems before using them.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?