Hi everybody!

Actually Studying in France , I am in my 1st year of Preparatory Class
for High school.

And one of the job I have to do is to make a Personal Investigation
around a topic i'd like to present in front of another people.

The fact is I need to find something who had a problem and who's been
resolved by an engineer. And I'd like to talk about the Modbus on it.
BUT the thing is I don't find any problems with the modbus. I can't
make a presentation about the interests of the Modbus, but I need to
find a problem around him. Have you encountered any problems during
the creation of this one? Did the modbus had major evolutions? If its a yes, Can you please say me more about it? Maybe we can talk about it on skype or i don't know? Thanks a lot if!

Thanks a lot too for reading this and take a bit of your time.
And thanks more if you take the time to reply.

<b>Moderator's Note:</b> It would be great if some of you would reply to this young man.

 

To me, the biggest problem with Modbus, is that Modbus data is not browsable - you cannot go to a device and discover that it has a few temperatures and cycle counters. All you can find is maybe 500 numbers, many of which might be utter garbage.

So bottom line, Modbus require YOU to know what the device it, then find a paper/PDF document from the manufacturer which explains that (for example) data number #73 must be divided by 320, then multiplied by 100.0 to produce a power "amperage" reading. Literally, if you do NOT have a manual, you CANNOT use a Modbus device effectively.

There is not much help for this in serial Modbus/RTU, but conceptually, with Modbus/TCP (Ethernet), one could use HTTP to query/GET some URL like "http://192.168.0.34/datadocs.json" and retrieve a machine readable data dictionary.

A few organizations have proposed solutions - an example is http://sunspec.org/ - which has machine readable specs for solar inverters, power meters, and so on.

This might be a good topic to study, especially if you can find examples of partial solutions.

- Lynn

 

Alexandre Wipliez,

MODBUS really has many different versions, and when MODBUS comes up in my work I usually have to ask how is it being used in this particular application (usage). And, to my mind there are really two basic uses of MODBUS: one is to allow different control systems to exchange information using a common format, and the second is to reduce the number of wires required to get information from field devices (usually sensors) into a particular control system.

Many process plants have two or more control systems, often from different manufacturers--which means the information in the control systems is usually manipulated and stored in different formats which requires different operator interfaces. So, in the plant control rooms there would be two or more (often many more) operator interfaces, each of which had a different way of displaying data in accepting inputs from humans (commands; setpoints)--which makes it difficult for the plant operators and their managers. They have to become familiar with and memorize different displays and systems and change from one to another to effectively control a plant. (Many plants are expanded and upgraded over time, and even some processes are automated over time--hence, the number of control systems in a plant can grow.)

By using a common method to exchange data between the various control systems and a common operator interface it is possible to aggregate (bring together) a lot information from two or more control systems into a single operator interface. And <i>as few as two wires</i> can be used to connect multiple control systems to one operator interface--or even other control systems, allowing multiple values from one control system to be used by other control systems to make process control easier and simpler with multiple control systems.

The second usage is similar but different. Since MODBUS is (in my opinion) a communication method, if sensors (switches, transmitters (devices that convert physical values like temperature or position or pressure into electrical values proportional to physical values) can be programmed to output their values using this common communication method which can be transmitted <i>on as few as two wires</i>. Using MODBUS multiple field devices (sensors) can all be connected together <i>using as few as two wires</i> to a single control system--or even to multiple control systems--to display and manipulate the values for process control or monitoring. (I think this usage of MODBUS is generally called MODBUS RTU, to differentiate it from "MODBUS.")

MANY signal values and commands can all be transmitted using MODBUS on <i>as few as two wires</i> and at some relatively fast rates.

So, MODBUS solves problems by, one, allowing various types of control systems, sensors and operator interfaces so communicate with each other, exchanging data and commands (Start; Stop; Pause; Increase; Decrease; setpoints; etc.) all using a very limited number of wires.

And there can be all manner of problems created by having multiple control systems, and needing to share the information between them, and/or to create a single, common user interface (commonly called a HMI (Human-Machine Interface), or to have an inexpensive method of connecting one or more field devices (sensors; etc.) to an existing control system or control systems or operator interfaces.

So, I would think your audience, if it doesn't already understand what MODBUS is and does, it would be helpful to do a little explaining/background on process plants and control systems and operator interfaces and the problem of getting multiple control systems and operator interfaces, and even sensors, all connected and talking to each other.

Before MODBUS, this was done using a minimum of two wires <i>for every signal to be shared</i> (exchanged), and this required printed circuit cards to output the signal and to accept the signal input. Consequently, the number of signals which were shared were kept to a minimum. And, again, there were quite often multiple operator interfaces in a plant control room or around a plant which were configured differently and couldn't display information from other control systems or plant areas. MODBUS allows the sharing (exchange) of LOTS of data commands between control system, including sensors, as well as being able to create a single, common operator interface <i>using as few as two wires</i>.

I hope you can see the number of examples and problems are endless. As process plants expand and grow, the number of various control systems in a process plant can grow--and usually,it's advantageous to
allow communication with and between control systems and the plant control room (where operators and their managers) want a single, common operator interface for all the various control systems in the plant.

And, that's just for process plants. I've seen MODBUS used in university experiments, and to connect weather sensors across a huge geographical area (even using radio-waves instead of and in addition to wires). It's used in electrical power generation applications, for monitoring and even control. Just about anywhere it's necessary for varied and different control systems and operator interfaces, and even sensors, to share (exchange) date and commands and setpoints MODBUS can be used. (It does have its limitations, especially in very fast processes, so it's not for every such problem--but certainly it works for many, many processes and applications.)

Hopefully, other respondents with much more experience in applying MODBUS and using MODBUS to solve communication and control (command) problems will offer the benefit of their experience. Maybe they can offer some drawings/sketches of before and after examples of using MODBUS to connect two or more control systems, or to connect multiple sensor to one or more control systems and/or operator interfaces. But, to me and my way of "simple" thinking MODBUS is a communication method which allows multiple devices and control systems and operator interfaces to all communicate with each other, using a minimum of wires (which makes the connection much less expensive).

I hesitate to refer to MODBUS as a "standard"--as much as it is a standard means of communications, it also has an extremely large number of very subtle differences. It's like automobiles--they're all forms of transportation, but they can take very different forms. That they can all share roads and be driven by drivers who speak different languages, they are "standard." But the differences between a Ford and and a Ferrari can be like night and day. An autobus (bus) is a way of carrying multiple people of all shapes and sizes and needs and purposes to many different areas using a single vehicle. Lorries (trucks) can carry multiple goods to many different places using a single vehicle.

Best of luck! I hope this helps a little bit--and it's not too simplistic. And I hope my analogies weren't too far astray or cause too much confusion.