OMNIfacts eNewsletter
  News from Omniflex
EndUser Electronic Edition
2004-05-19 00:00:00


Omnifacts is published by Omniflex Pty Ltd. This newsletter is designed to inform and educate in the areas of monitoring and control.
This publication may be freely redistributed if copied in its entirety.  Portions of this newsletter may be
reprinted with permission

2 wire transmitters and 4 wire transmitters. Why?
Ian Loudon

The question has many answers so lets start with what they have in common.

The both have a sensor side(Input) Thermocouple or RTD or millivoltage and they both have an output side (Transmitter). The difference is in how you power it. The 4 Wire transmitter has a pair of power wires to it whereas the two wire transmitter uses its transmitter output to receive its power.


The standard transmitter signal in industry is 4-20mA chosen for its ability to represent small transducer signals from the field. It has the following advantages:

Live zero i.e. 4mA output for minimum input signal. If the unit fails the output reads zero therefore it has a safe predictable failure mode. It can be used to transmit signal long distance by simply applying ohms law. Long distance = more cable resistance so more voltage is required to keep the current the same.

The differences

Two Wire Transmitter

A Two wire transmitter always has only one type of output signal option a current loop of 4 to 20mA.

The unit must power itself from less than 4mA. Low power device.

It nearly always is located close to the sensor, particularly those with small electrical signals like thermocouples and RTDs.

Less wiring is required you save on a pair of wires to the field.

Two Wire transmitters lend themselves to being suitable for Hazardous areas where danger of ignition may occur.

Four Wire Transmitter

A four wire transmitter can have any output type, Voltage, Current and even bipolar outputs which allows reverse action in control applications.

The unit is powered separately and can consume as much power as it needs for its function. This is particularly useful for various types of output.

It is normally located at a suitable point away from the sensor  and close to the control equipment

Requires more wiring so it tends to be applied close to a power source, typically and interface panel.

Not suitable for Hazardous areas but may have Intrinsically safe inputs to allow them to interface with Hazardous areas.

Maxiflex Router/Repeater Solutions


The Implementation of Maxiflex Network Interface Modules (NIMs) and routing technology has added greatly to the flexibility of Maxiflex Networking capability. Using NIMs it is possible to create on the traditional Conet BUS type network Topology the following:

A Conet Repeater

A Conet T off

A Conet Star

Conet with its ability to run up to 10km on a screened twisted pair cable has significant advantages over other fieldbus networks however the addition of NIM technology has added an unprecedented level of flexibility to Omniflex Networking Solutions.

The Repeater provides two Conet Networks on either side(Back to back) which are electrically isolated from one another but will function like a single Network. The repeater allows cable extension or enhances the Conet Network where cable limitations are experienced by dividing the Network into smaller segments resulting in better electrical characteristics on the cable segments..


The T-off provides 2 or 3 Conet Networks allowing a T junction in the Network Topology. A classic example of this is the vertical mine shaft with horizontal shaft. A t-off at each level allows the Conet Network to service each level minimising cable runs.


The Star allows multiple Conet Networks up to 16 to radiant out of a central point taking the shortest path to the field from say a control room. This topology is ideal for the Control room situation where Networks radiant out into the plant in all directions


NIM Technology is not restricted to Conet alone. Modbus, Third party Protocols and Ehernet can be integrated very simply providing complete top down flexibility in Plant Networking Technology. Talk to us about your Network challenges.

Omniterm TFX Universal Special Function Module

  • Select from a wide range of computation functions
  • Fully configurable by the user
  • Accepts two 4-20mA or 1-5Vdc inputs
  • One universal 0-20mA / 0 to +-10V / 0 to +-10mA or
    Output isolation  1500Vac

The OMNITERM TFX Special Function Module is ideal for those special applications where mathematical computations are required on your analogue signals. The TFX accepts two 1-5V or 4-20mA inputs and provides one universal output.

The output can be configured for unipolar or bipolar outputs of current or voltage from 1mA to 10mA; 0-20mA; or 1V to 10V, as well as pulse outputs for totaliser and frequency translation applications.

This module uses advanced state-of-the-art digital measurement techniques, combined with extremely user friendly software configurability for best ease of use without factory required setup when ordering.

Full output isolation to 1500Vac ensures trouble-free installations.

Last Month:

Last month we talked about several important topics including:

  • 2 wire transmitters and 4 wire transmitters. Why?
  • Maxiflex Router/Repeater Solutions
  • Omniterm TFX Universal Special Function Module

If you missed these or other key discussions,
you can find the back issues on the newsletters page of our website:


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This publication may be freely redistributed if copied in its ENTIRETY.  Portions of this newsletter may be reprinted with permission.

(c) Copyright 2003 OMNIFLEX PTY LTD 

In this Issue
2 wire transmitters and 4 wire transmitters. Why?
Maxiflex Router/Repeater Solutions
Omniterm TFX Universal Special Function Module
Last Month's Issue

Omniflex Transmitters

Two Wire

Four Wire

Intrinsically Safe

Designed for SIL Compliance


Two Wire Transmitters

Powered from 12 to 45V

Certified Intrinsically Safe for application as both barrier and for location into the hazardous areas.

Four Wire Transmitters

24 V dc Powered with Universal Inputs and Outputs in one unit does all. Minimize spares holding. PC configuration port allows online diagnostics and field proving of loops.

Intrinsically Safe Inputs.


Conet Repeater

Extend Conet Network beyond 10km.

Improve Network Communications with segmentation on marginal cables

Implement T-offs from the Main Conet BUS.


Conet Star

Create Conet HUBs with multiple Conet Networks saving on cable runs and improve performance. Mixed Topologies with Star, T-off and Mix communication media and communications protocols.

Ethernet Options

Create connections from Conet LANs to Ethernet using the same Routing Technology simply and easily. Construct flexible Remote I/O applications and Integration of legacy control and monitoring devices into Ethernet reaching all those hard to get at sites on the plant.


Omniterm TFx

Minimize Live Cycle costs.

Easy Configuration

Suite of Math Functions

High Reliability


Mathematical Functions

The Omniterm TFX comes equipped with a wide selection of common mathematical functions. When combined with the user configurable 32 point linearisation table, this powerful computation module can be used to solve a wide range of signal processing applications such as belt slip monitoring, dual-redundant analogue inputs, ratio monitoring, rate-of-change monitoring etc. etc..