Fieldbus & Industrial Networking

How to fix those plant signal ­interfaces

March 2016 Fieldbus & Industrial Networking

The ubiquitous problem common to every plant is the interface of plant measurement signals to the monitoring and control systems. Unfortunately for many plants this is the single biggest area of weakness and with the success of the organisation depending on these measurements more attention should be afforded to the integrity of signal conditioning systems.

The problems faced by these systems are numerous and include:

• Aged cabling and interfaces.

• Long cable runs.

• Earth loops.

• Interference from other plant devices.

• Floating earth potentials.

• Isolation of signals from PLC, scada and DCS.

• Legacy instrumentation.

• Isolate grounded equipment.

• Adding instruments to existing loops.

• Poor design.

• Converting current loops into accurate 1-5 V.

• Protecting against open circuit loops.

• Load dependency calibration.

The consequences can be onerous to the bottom line when these listed costs start adding up: damaged PLCs and DCS front end; downtime losses; inaccurate readings which affect plant production; reactive plant troubleshooting and increased maintenance load.

Many well informed plant owners and system integrators fit signal conditioning interfaces as standard during the design phase – the increase in cost on the signal interface more than pays for itself later in productivity and reliability. This is often sacrificed and traded off in the initial design.

Omniflex has accumulated 40+ years of experience in signal conditioning system design and has produced many innovative products in this particular field. Retrofitting this to plant is easy with DIN rail mounted modules which can be located in marshalling panels or termination boxes. A compendium of common plant interface problems is addressed in the following tutorial: “Loop powered isolators are a cost effective solution to solving loop problems”.

Interested readers can download the full tutorial at:

For more information contact Ian Loudon, Omniflex, +27 (0)31 207 7466,,


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