STANDARDS & PROTOCOLS
HART increasingly used for quick diagnostics Pluggable disconnect terminals for process engineering control systems By Alan Sappe’ and Torsten Schloo
Figure 1: In the chemical and process industries, the cost of technology is rising, especially given tighter process schedules — pluggable test disconnect terminals simplify servicing and maintenance. All graphics courtesy: Phoenix Contact
“M
ore complex, more compact, more efficient” – is the credo for control system engineers in the oil & gas, chemical, and process industries. This view also accelerates efforts to squeeze production into tighter and tighter process schedules. Over the last few years, engineers in the petrochemical and process industries have tested many new ways to make production quicker, cheaper, and more flexible — quite successfully in some cases. These advances help the industry recover relatively quickly from cyclical downturns. Modularization, industrial automation, retrofitting, and system expansion all contribute to industry success. These tendencies have also facilitated the increased use of Highway Addressable Remote Transducer (HART) communication protocolcapable sensors and control valves. This protocol uses traditional 4-20 mA current loop as a transmission medium for a modulated data transmission signal in accordance with the Bell 202 FSK standard, where communication is conducted with signal sequences of varying frequencies alternating between command and response on simple twisted-pair cables. This keeps implementation costs down, without affecting the actual analog measurement signal. It allows the system
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to transmit multiple pieces of information and makes it possible to diagnose problems or configure devices remotely. Over the past 25 years, countless HART devices have been installed in applications related to flow, pressure, temperature, and fill-level measurement. In many cases, only the 4 - 20 mA signal is permanently connected, while HART data is only accessed during start-up or troubleshooting. What are disconnect terminals? HART is used with several sensors for measuring temperatures, pressures, vacuums, filling levels, distances, conductivities, densities, and pH values. Valves, servomotors, inputs and outputs of all varieties, and analog values are also controlled using HART. This can be done in three ways: 1. Readjustment and calibration of measurement values 2. Troubleshooting in complex systems 3. Parameterization or querying of device configurations using a computer. It is often necessary to connect measuring or diagnostic devices to a current loop — to measure the parameters that the sensors receive and to evaluate the switching states or system parameters. In conventional control systems that do not use a current loop for data communication, a short signal interruption does not necessarily cause any problems. But if the current loop is also the transmission medium, interruption causes a break in communication with the HART modem and a system error message is sent. That explains the indispensable value of interruptionfree connection for HART-controlled systems. Innovative and rugged disconnect terminals have test sockets on each side of the disconnect point where a measuring device is inserted before the disconnect knife or switch is opened. This action must be completed before the measurement. Test disconnects make that action easier for technicians because the disconnect zone is
