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Modbus General Description

Thread Starter: smiffy Started: 12/15/2011 6:41 PM Replies: 25

5/6/2012, 1:08 PM

gre_m

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modbus exist on rs422/485/232 as add on over ptp-communication and add-on over ethernet/TCP=modbus TCP. Profibus - is all-sufficient prtocol, therefore modbus as add-on over profibus not exist.

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5/6/2012, 2:05 PM

smiffy

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I think they were hinting that I could do a similar document to explain the basics of Ethernet and Profibus (and maybe even Profinet). I don't think that they were suggesting that modbus can be setup over a Profibus interface.

I will see what I can do to come up with Ethernet and Profibus documents. However, these things tend to take quite a bit of effort and time. I will also need some proof reading services from my fellow experts.

Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots.

So far, the universe is winning...

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5/6/2012, 4:02 PM

Arunraje

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Ya Smiffy please do the needful...

Regards.

"Nothing is permanent, except the CHANGE"

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5/6/2012, 8:08 PM

AUTOMATION_BOUHA

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Thank u SMIFFY thats nice from u, that will help us more, and i hope that we don't bother u Rnf:lblSmileyTooltip_

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5/7/2012, 1:11 PM

Aniket S Dange

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Dear Arun,
this will surely help you..
http://www.plcmanual.com/isoosi-model
Enjoy..

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Arunraje

7/20/2012, 4:00 AM

YvonV

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its a good read. thank you for that effort.

Yvon

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Thank you for this post from:

sorooshi, smiffy

9/29/2012, 9:21 AM

Hunky

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Thank you for the support.

I am a little confused here.

I am doing a project where in i have to communicate with ABB DCS with CP 441-2. The following is the requirement.

Data from DCS to PLC

Analog Output - Address : 400001

Digital Output - Address : 00001

Data from PLC to DCS

Analog Input - Address: 300001

Digital Input - Address :100002

In the above circumtances,CP 441-2 shoule be acting as Slave / Master?

If it is acting as a Slave will it read the data from DCS.

Thanks for your support and co-operation.

MIA

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9/29/2012, 12:37 PM

Kaulquappe

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Hello Hunky,
the used addresses do not imply any information which communication partner is master and which one is slave. It depends on the application on site. The master is the active one, it can read data from and write data to the slave. The slave can't initiate communication, it can just answer the client's requests.
You have just to ask whether the DCS acts as master or not.

Regards,
Kaulquappe

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dchartier

9/29/2012, 12:42 PM

dchartier

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Hunky wrote:

Data from DCS to PLC

Analog Output - Address : 400001

Digital Output - Address : 00001

Data from PLC to DCS

Analog Input - Address: 300001

Digital Input - Address :100002

In the above circumtances,CP 441-2 shoule be acting as Slave / Master?

Hello hunky;
It is indifferent which of the 2 (CP 441-2 or DCS) is master on the bus, especially if you have only these 2 nodes on Modbus RTU.
Choose one to be master based on ease of programming and familiarity,
If you select the DCS as master, you will have to program the Modbus jobs in the DCS, and choose:
- Write multiple coils (Modbus function 15- write to the 0000x outputs of the PLC)
-Write multiple registers (Modbus function 16- write multiple 4000x registers to the PLC)
-Read input registers (Modbus function 04- request 3000x registers from the PLC)
-Read discrete insputs (Modbus function 02- request 1000x inputs from the PLC)
Of course if the roles are reversed you would have to select the Modbus jobs in the Cp 441-2 so you would write digital and analog inputs to the DCS, and read digital and analog outputs to the PLC.

The slave will only reply to the master when Modbus jobs are sent to it, so everything is to be planned from the master's point of view.

I do not know how difficult it is to use Modbus RTU on your DCS, but it might be easier to let the CP 441-2 act as a slave, there would very little programming to be done on that side. Have a look at the following sample program for Modbus slave functions proposed by member Amit recently:
https://www.automation.siemens.com/WW/forum/guests/PostShow.aspx?HTTPS=REDIR&PostID=359580&language=en

Hope this helps,
Daniel Chartier

=== Edited by dchartier @ 9/30/2012 5:22 PM [GMT ] ===

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10/3/2012, 12:16 PM

Saggy