MODBUS

Articles on both these topics can be found in Wikipedia, and should remove all confusion.

If, after you have done as PWSlack suggests and read the pieces, you are still confused, come on back.↑

yeah i am back........

Did you understand what you read?

Can you summarize?

i just got that all this stuff is merely a business strategy

• "i just got that all this stuff is merely a business strategy"

LOL, so wrong, and yet so right!

They are communications protocols and standards, both based primarily on RS-485 hardware standards. But Profibus operates at much much higher speeds and has significantly more error checking than Modbus does. Modbus RTU speed is 9600 to 115kbps, whereas Profibus is 500kbps to 1.5Mbps depending on conditions.

The thing is that Modbus, originally developed by Modicon, is now an open protocol standard that is "free", meaning no licensing requirements for those who want to implement it. Profibus is not, so in order for an equipment mfr to implement it, they must buy a license from the Profibus Industry (PI) association and everything that connects to is must be tested and certified. People often ask why it is that even though they both use the same hardware standard, the Profibus rate is so much higher, that's why; untested devices using Modbus on a network can cause data integrety issues that force repeated transmissions to get through, and that problem multiplies exponentially with speed, hence the limit at 115k. Profibus devices must be tested to not cause issues and if they fail, they don't get the certification.

So for a device mfr to want to implement Profibus, they must know in advance that they are going to sell enough of their devices to Profibus users to warrant the cost, versus with Modbus all they have to pay for is implementing the communications packets to fit the protocol standard.

If you are an end user thinking of which one to implement, Modbus is OK only for things that don't need to communicate much or often. Under adverse conditions, I have seen simple commands take 15 seconds to implement. That can be a process killer. But sometimes your device has the ability to take immediate action locally, and all the communications are doing is letting some higher level system know what happened and when. In that case, Modbus is often all that is necessary.