Hi

Simply, i am trying to send a command to a device (Slave) using modbus RTU from my S7-1500 (Master). As per the device manual, in order for the command to be executed, a register has to be sent along with its ones complement using the same write command. the question here is... how is this achieved ?

Thanks

Hello marvin;

I am not sure I understand your question.

First, is this a Modbus RTU (serial, RS-232 or RS-485), or a Modbus/TCP project?

The manual you refer to describes the S7-1500 master, or your slave?

To my knowledge, to write to multiple registers you must use Modbus function 16; that way you can access up to 124 regiters of the type 40000 in the slave; so you must specify to the master which slave he will access( the slave address), which Modbus function he will use, how many registers or bits he will read/write, starting at what offset in the «modbus register adrtesses,

See how it is done in the relevant sample projects that follow:

Tips : S7-1500 – Modbus RTU

https://support.industry.siemens.com/tf/ww/en/posts/104271/

How do you program and parameterize Modbus/TCP communication between S7-1500 CPUs and S7-1200  CPUs?

https://support.industry.siemens.com/cs/ww/en/view/102020340

Hope this helps,

Daniel Chartier

Hi Daniel

Am using MODBUS RTU on RS485, and for the CPU, 1215C is been used.

The manual i mentioned is for the slave.

This is what the manual says: 

How is this done ?, any examples using the MB_Master block ?

Hello marvin;

To setup Modbus RTU on a S7-1200 CPU (as a master), you need 2 instructions:

- MB_Load: sets parameters for communication (baud rate, parity...)

- MB_Master: sets the parameters to exchange data with a slave

I have attachjed an extract form this manual that explains how MB_Master parameters are used:

S7-1200 Programmable controller

https://support.industry.siemens.com/cs/fr/en/view/36932465

You select the Modbus function you want to use as part of the MODE parameter; Function 16 is MODE= 1, or 2; see page 4 of the document i attached. Then you prepare the data to be written inside a DataBlock that you refer to in parameter DATA_PTR,

.See the examples I proposed in my earlier reply for more details.

Hope this helps,

Daniel Chartier

Hi dchartier

your information has been very helpful (Thanks), i almost got the idea but i'm stuck with one more thing which is, how do you make the MB_MASTER read two addresses from the data block, while configuring the DATA_PTR i am only able to select one row from the data block, not two. In my case i need to write to two different addresses using the same write command.

Hello marvin;

Have you looked at parameter Data_Len in the MB_Master instruction?

Data_Len:

Data Length: Specifies the number of bits or words to be accessed in this request. See the Modbus functions table below for valid lengths.

Normally I would expect that by setting the parameter value from 1 to 123 (the acceptable values for Modbus Function 16) that you could access that number of registers.

Hope this helps,

Daniel Chartier