Hello,

I have S7-1200, I setup modbus rtu communication with modbus slave device. It work fine using last library "modbus_comm_load" and "modbus_master".
But I can read only contiguos register, I set "start address" and "data length". But If I need read for example register 40001 to 40010 and 40500 to 40550, how I can do ?

Can you help me ?

Thanks

M.

Hello,

very simply. Your DATA_ADDR and LEN parameters can be set as you require. Due your <data type> on Slave you can change DATA_ADDR as start register address and LEN as amount of registers you want read. To this situation you need have set enough long receive buffer.

 Hello amatteo;

In that case, call the Modbus master block twice for the same slave, once for addresses 40001 to 40010, another for addresses 40500 to 40550.

It is tha same procedure if you wanted to address a second or third slave from the master. Just change the calling parameters of the Modbus m,aster within a new call.

Hope this helps,

Daniel Chartier

amatteo78

Hello,

I have S7-1200, I setup modbus rtu communication with modbus slave device. It work fine using last library "modbus_comm_load" and "modbus_master".
But I can read only contiguos register, I set "start address" and "data length". But If I need read for example register 40001 to 40010 and 40500 to 40550, how I can do ?

Can you help me ?

Thanks

M.

Hello,

I had think I could call block more time, but I know I can only once for time. How I can check when first block finish to start with second, and then again first.... and so on....

Thanks

M.

Hello amatteo;

Excellent question!

When the Modbus_Master has finished executing, the DONE bit is activated (for one cycle only).

By checking the status of the DONE bit you can know when to start another request of the Modbus_Master block.

Check the sample program described in the following Application Example:

https://support.industry.siemens.com/cs/ww/en/view/47756141

Hope this helps,

Daniel Chartier

dchartier

Hello amatteo;

Excellent question!

When the Modbus_Master has finished executing, the DONE bit is activated (for one cycle only).

By cheecking the status ofthe DONE bit you can know when to start anpther request of the Modbus_Master block.

Check the sample program described in the following Application Example:

https://support.industry.siemens.com/cs/ww/en/view/47756141

Hope this helps,

Daniel Chartier

 Thanks, very good docs.... I follow it for config "step logic"

M.

When using the write function instead of the read do you also have to have a Seperate Modbus Maste block written in the same format? 

So in this case you have two to read different addresses but if I wanted to read the registers then write to some , would it be a case of 2 modbus master blocks, one reading then once done the other activates a write? 

Thank you

0_Einstein

Multiple "Multiple read register" calls, one with start address of 40001 [0], length 10, and the other with start address 40500 [499], length 51.

You can only have one active at a time, so disable the first one when performing the second one.

Or, another way to think of it, reconfigure the slave parameters with different start address / length values every other request for new information.

Another choice is to have a much larger buffer DB to accept remote data to so you can make a single call, start at 40001 [0], length 649.  This last one may cause problems because you cannot request that many at once according to the documentation.  So, you would be back to the original suggestion. 

amatteo78

Hello,

I have S7-1200, I setup modbus rtu communication with modbus slave device. It work fine using last library "modbus_comm_load" and "modbus_master".
But I can read only contiguos register, I set "start address" and "data length". But If I need read for example register 40001 to 40010 and 40500 to 40550, how I can do ?

Can you help me ?

Thanks

M.

 

" another way to think of it, reconfigure the slave parameters with different start address / length values every other request for new information."

Do you know exactly how to do this in reads to reading and writing on a register? Do you need to create an additional slave module with the write registers to correspond to an additional master module with the write mode activated? 

Hello Edward;

Replying to that question  in a way that is bot easy and effective requires that you have a background in the Modbus protocol specification. Since Siemens is implementing Modnbus with ts own functions and hardware, they are a bit free with the specifics, but they will try to program it so it reflects Modbus basic functionalities.

For a refresher (or introduction) to Modbus protocol, I often refer to this document prepared by our friend smiffy, it has helped many in this forum dealing with Modbus RTU and Modbus TCP:

Modbus General description:

https://support.industry.siemens.com/tf//ww/en/posts/modbus-general-description/66876/?page=0&pageSize=10

Tell us if after studying this short presentation you still have doubts, we will see if we can provide more specific details.

Ho^pe this helps,

Daniel Chartier