Hi everyone, I read about modbus a lot in here thank you for your sharing and interest but I can figure out how to giving address.

In my project I have used s7-1200 PLC with cm1241 for modbus rtu and it will be a slave.

I have prepared a data block for modbus which contains both integers and bools. Some of bools will be read and some of them will be written by the master. Samething for the integers some of them will be written and some of them will be read by the master.

Until here, everthing seems ok but I can not figure out how to mapping address for datablock because, I don't know how to give an address for reading the integer, writing the integers, reading bools and writting bools which are different.

I will add picture of datablock that I will be used for modbus block.

Please someone may explain me clearly, there is only one datablock and how they represent different address;

coil start  00001-09998 read/write

Discrete inp. 10001-10998 read only

input register 30001-39998 read only

holding register 40001-49998 read/write

extended holding register 40001-105536 read/write

I have heart that modbus work with 16 bits and address has to be represent 16 bits as word or int. How do coils or discrete input works on modbus???

My last and summary question;

What should I give the master as address to connect to my datablocks?

Yusuf_TUNÇ

I will add picture of datablock that I will be used for modbus block.

Please someone may explain me clearly, there is only one datablock and how they represent different address;

coil start  00001-09998 read/write

Discrete inp. 10001-10998 read only

input register 30001-39998 read only

holding register 40001-49998 read/write

extended holding register 40001-105536 read/write

...

My last and summary question;

What should I give the master as address to connect to my datablocks?

Hi.

Have you seen this FAQ yet?
https://support.industry.siemens.com/cs/ww/en/view/47756141

A picture borrowed from the System Manual...

For your case the MB_HOLD_REG := "MODBUS DATA"....
or P#DB??.DBX0.0 WORD ??  where ?? is for your case.

The trick is that you do not have any control on what type data the SLAVE makes available. It is only holding register. The transaction is only concerned about transfering WORD data. The MASTER should then decode the raw data from the registers to bit arrays and INTs.

So if the MASTER wants bit 12.0, it will need to read register 40_007 and extract the bit itself. Reading "F76 CP 1 SPEED" will require reading 40_009 and so on.

This Modbus RTU concept only gives you a way to transfer raw data, where the data is linked to register numbers from 40_001 onwards.

Does this help you?
w

Hi, Wiliam. Master, the other campany want me to give them address to control and monitor my system so I need to give them a modbus list;

coil start  00001-09998 read/write

Discrete inp. 10001-10998 read only

input register 30001-39998 read only

holding register 40001-49998 read/write

What should I do,

Is it enough that giving address 400001 to 400..last word?

If I give them only 40001-40... address, can master do both write and read?

Thanks for your interest but I need more detail please.

Yusuf_TUNÇ

Thanks for your interest but I need more detail please.

Yusuf,

^{It is hurtful to read those words.
It is as if you expect a volunteer to hand you a solution.
What specific is it that you need?}

^{Please know I am trying to take my best guess
without knowing the depth of your background on this subject.
This remains far from a solution and needs your interaction.}

Your information is not wrong.
The Modbus RTU protocol handles the elements you mention
                     - and even more.

Modbus RTU Slave protocol is only partially implemented in the S7-1200. Only the read and write of registers 40001 ++ is implemented. There is nothing else you can do, unless you create the coding to implement more Modbus functionality. The PLC point-to-point commands makes it possible but it is not worth the effort. I recommend to make work what you have.

I have worked with Modbus RTU slaves where I could only use the holding registers. It is not unheard of to have only holding registers. The WORD registers provided bits, UINT values or INT values. It is common of to have the Master need to extract bits from WORD data or convert INT values to REAL numbers by using other INT values as multiplier and divider.

..now I am jumping ahead too far -
Co-ordinate with your communication partner to find what is possible for them. If the Master is programmable, the data can be read and written with what you can supply.

Your coding only links (maps) the Modbus register numbers to the PLC memory. The Master then tells the Slave which register it needs to link to and if it needs to read or write. There is not much else for you to do for the communication.

On the synchronisation of your internal data and that of the communication data there is much clever thinking you have to do. It does not have to be complicated. Planning and organization is key. When it comes to bits - for example - we normally make split between CONTROL and STATUS. A bit (and even values) should not be both.

Make sure that you do not reference (read) in your code the values in "Modbus Data" that is suppose to be read-only for the Master. So if the Master do write to those registers, the next cycle you update those with your own values.

----

I am concerned about the amount of data you showed us in your picture. I hope that the Master is not going to attempt to access all of those at once. Modbus RTU is not a fast protocol. Ethernet based comms (like Modbus TCP) would be better suited.

I hope this chat is of help. Please feel free to spell out what issues / fears you have as some might help to brainstorm with you. The more specific you are, the more helpful the Forum can be.

Greetings.
w

William you are perfect thank you very much.

Ok I just give them my datablock list and I will tell them that is start with 40001. Some of the address should not be written by the master so I will take the precaution. Therefore the address will be only readable.

Next will be comminication with them will be problem but I have time to do. Now, I need to test physically the modbus whether works or not.

Really really thank you.

Yusuf_TUNÇ

William you are perfect thank you very much.

Ok I just give them my datablock list and I will tell them that is start with 40001. Some of the address should not be written by the master so I will take the precaution. Therefore the address will be only readable.

Next will be comminication with them will be problem but I have time to do. Now, I need to test physically the modbus whether works or not.

Really really thank you.

... I am yet to meet the human that is perfect (wink + smile).
Thank you for your positive feedback.

Please expand on your thoughts -
    communication with them will be problem

With you being the Slave you make available for them the data they can take. The status portion is thus not a problem.

For the control, you can have mirror data used as buffer. When the buffer and comms data is not the same, you can take action. Consider also to verify the data to be legal before using it.

I have seen systems where you give for example a speed setpoint, but the value is only used when a bit is triggered to say "use new value". This type of handshaking makes your task much simpler but also ensure that all relevant data is in place before triggering the task.

Your thoughts?
w

First my project had only S7-314 PLC, TP1500 and PC station. It is about to finish the project customer demand to add modbus for alarm monitoring system (For vessel).

Therefore our campany disegner adds CP341 and purchasing manager buys CP341 without dongle and modbus library because of they have no idea bout it. It took my more than two weeks to learn dongle and library that I have to buy. After all, It was too late to convince the customer pay more money. So decide to add S7-1200 PLC and cm1241 those are cheaper than s7-300 dongle and library price.

Of course, exchange data beetwen s7300-s7-1200 and now modbus.

I would try to use Modbus tcp/ip without module but I have learned that it is not free, too.

Still I am not sure that  I am doing right. By the way there may be more easy way.

Thank you for helping.

Yusuf_TUNÇ

Of course, exchange data beetwen s7300-s7-1200 and now modbus.

I would try to use Modbus tcp/ip without module but I have learned that it is not free, too.

Still I am not sure that  I am doing right. By the way there may be more easy way.

Tomorrow I will contact the other company which will be master.

Thank you for helping.

Yusuf - what an ugly challenge you have!
Thanks for colouring in the picture and making it clear.

(sad smile) For what it is worth - the S7-1x00 offers many communication methods without the licensing issue. Modbus TCP can be done over the Profinet port and the same port can communicate with the S7-300 too.

Yes there are other methods available. A protocol converter is also a possibility. One such is a more all-in-one that you can use as HMI too (Red Lion).

But I think that I must call upon our expert to weight in his thoughts on the matter.

S'il vous plaît monsieur Daniel Chartier - What would the options be to get data from an S7-300 to another system.. most possibly a Modbus RTU Master. Merci beaucoup.

w

New question published by David4444 is split to a separate thread with the subject how to organize the Register in the slave - Modbus RTU slave s7-1200.

Best regards
Jen_Moderator