low cost connection for up to 30 HART field devices via a single Maxiflex HART module
Feb 2000Analytical Instrumentation & Environmental Monitoring
Omniflex
For the vast number of HART enabled smart devices now installed in plants around the world, Omniflex has developed a network interface module (NIM) which enables either two analog loops or 30 digital loops to be connected for data acquisition, control and calibration. Expansion is easy as each Maxiflex CPU can handle up to 15 NIMs (30 analog or 450 digital HART loops) and up to 126 Maxiflex CPUs can be connected to a Conet LAN.
HART is a de-facto global standard for enhanced digital transmitter communications and petrochemical and other plants typically have thousands of HART devices measuring flow, pressure, temperatures, mass, mass flow and other process variables which are required by the DCS or scada for monitoring and control. HART uses frequency shift keying to superimpose digital communications at a low level on top of the 4-20 mA signal, enabling 2-way communications to take place, thus making it possible for additional information beyond just the normal process variable to be communicated.
As HART is a master/slave protocol, each Maxiflex HART NIM acts as a primary master that provides a permanent host interface for two HART networks simultaneously. These could be two single analog (4-20 mA) loops or two groups of 15 digital loops operating in Multidrop mode. The NIM provides full 1500 V r.m.s. isolation to the field to maintain integrity of signals in the presence of ground loops and electrical noise on the plant.
Because of the spread-out nature of many plants, it becomes very expensive to connect HART transmitters to the DCS and as a consequence, many of these will remain as standalone devices in remote corners of the plant, requiring operators to perform manual readings and regular calibration. Because Maxiflex and Conet offer a low-cost, long distance data acquisition solution, the HART NIM becomes an attractive, cost-effective option to integrate a wide range of instruments into the main process control system.
HART multidrop networks are ideal for remote monitoring stations, tank farms, pipeline distribution systems and other monitoring applications in which fast data rates are not required. Because there are so many HART devices available on the market, saving millions of man-hours that would otherwise be spent on calibration and resultant plant errors due to incorrectly set up equipment, integrating these devices into the mainstream process control system is therefore a logical move.
For ease of use, the NIM is also self-configuring and on power up, searches the HART networks to build an automatic inventory of connected devices found. Thereafter various data elements are continuously and automatically read from the devices and stored in the NIM's data interchange table (DIT) for use by the rest of the Maxiflex System and also by the scada or DCS.
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