4 wire 3 wire 2 wire signal modes
Process parameters are signalled to control panels or monitoring devices in various ways . Electric form of signalling are common in instrumentation field .You can see various methods, wiring concepts,protocol etc.This article explains one such classification . signal transmission is differentiated as 4wire and 2wire transmission ,3wire , on the basis of the number of wires used for this transmission .
Requirements of signal transmission
power
- A device like Transmitter or sensor needs power for its functioning .They works by this power .with respect to a device This power is not a variable .It is a requirement .For example to turn on a TV in our house what we need? We need power supply. And it should be within the specified limit .Like this we need a power supply for a Transmitter or devices to turn it ON .
Input variable
Transmitter or sensor is meant to measure and transmit some variable in the process .in process control plants we can see lots of parameters which we need to measure and control . process parameters like pressure , temperature,level,flow etc are measured by this transmitter .These process parameters are the inputs .
Output signal
How the Transmitter inform the value of a process variables to a control room or control panels or a receiving station .How? . A popular method is transmitting the process variables as a current signal .For example take a case of a pressure transmitter .It transmit a current signal proportional to the pressure input .so a monitoring station receiving this signal can interpret this signal value . In current transmission method ,4-20 mA standard is used .so this signal is the output of the transmitter .
.So power and signal,two important parameter .These power and signal requirement of this transmission channel is established through wires / cables .copper is mostly used .we know for powering a device we need two wires . +Ve and – ve .And for transmitting a current or voltage signal we need two wires .+ve and – ve .So basically we need four wires .Two for power the device and two for transmitting the signal .
.Systems designed such a way that these need for seperate wires for powe and signal not needed. Some of the lines can be designed to carry both power and signals Based on the number of wires used the transmission is categorized as. 4wire ,3wire ,2wire signal . Let us discuss one by one
4-wire method
In this method four wires are used to manage power and signal . Signal and power is carried by seperate wires .
See below figure . A 4 wire method of signal transmission techniques is shown .A four wire loop is accessed by a control panel or a receiving instrument is shown . Current is measured in series .This is achieved through a current measuring circuit by the control system .
2 -wire method
In this method two wires are used to manage power and signal . Popular signal standard is 4-20 mA .Signal is transmitted as current .
See below figure . A two wire loop is accessed by a control panel or a receiving instrument is shown . Current is measured in series .This is achieved through a current measuring circuit by the control system .
3- wire method
Here three wires are used .see the below figure .Out of three wires one wire is common to the supply and signal .
A vibration proximitor is connected to the control panel is shown .Here out of the three wires .one is +ve one – ve and the third one is for signal output .
See power to the device is fed through -24 V and common terminals. And signal is measured across common and output terminal .
