How to choose communication interface and protocol

2019-11-15 16:58:00
Stefan
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Most of our products come with :

Communication interface: RS232, RS485, SDI-12, 4-20mA for your option.

Communication protocol:  NMEA0183, MODBUS-RTU, SDI-12 for your option.


Part I. Communication Interface

We're not going to discuss the fundamental communication principle between RS232, RS485, SDI-12, 4-20mA.

But the advantage or disadvantage of them.

1. RS485: 

Max. cable length:1 km (3200 ft);

You only need to take care of two cables for signal transmission:  DA+ and DB-, which makes the installation pretty easy.


2. RS232


Max. cable length:6.5 m (20 ft)
You will have to take care of three cables for signal transmission:  TXD, RXD and GND for signal, which makes the installation a little bit complicate



3. SDI-12

SDI-12 interface was originally designed to sharply reduce power consupmtion of sensors long time ago.

With improvement of technology, many instruments can easily realize low power consumption while maintain their excellent performance.

Therefore, most people choose this interface mostly for purpose of making sensor be compatible to their data logger.


4. 4-20mA

4-20mA output is an analog output.

The lower and upper current output respectively correspond to lower and higher range of measurement.

Take wind speed output for example. We can use 4 mA to represent for 0 m/s and 20 mA to represent for 60 m/s.

4-20 mA was once widely used in traditional data logger.

Each parameter will have to occupy two cables current loop, which will require plenty of cables for mutiple parameters output.

So it's unrealistic for multiple parameters, in that case we prefer using RS232 or RS485 output.


Part II. Communication Protocol


NMEA0183 Protocol

NMEA 0183 is a combined electrical and data specification for communication between marine electronics

This is an active output protocol, message is output in format of ASCII.

Example below for your info.


$WIMWV,180,R,0.01,M,A*06


Explanation:

$WIMWV:NMEA Wind Protocol Header


,:separator between each two parameters

180: wind direction in °

R: relative wind

0.01: wind speed

M: unit of wind speed, which stand for m/s

A: data frame is valid

*06: checksum, 


MODBUS-RTU Protocol

Modbus protocol is a polled and answer protocol.

You send a request command to inquiry value of parameters stored in registers.

And the instrument respond you with regularly updated value of registers to you.


The biggest advantage of this protocol is that you can assign different address to every instruments connected to RS485 bus line.(up to 256 address, 00-FF).

The inquiry command must include an address to indicate with whom you want to communicate.

Only the device assigned as that address will be allow to respond to your request.


By time-divided communication, you are able to access all data from all sensors connected to same bus line.

How wonderful!


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