hi Martin_St81,

where exactly is the problem?

Write the information you need(inputs of your slaves) to the register you send, and read this register in the Master. Then you can use them you control the outputs of your master.

hope this helps,

regards,

You are asking a very broad and open-ended question, similar to how do I drive a car? This is very difficult to answer.

You need to ask specific questions relating to your needs.

If you do not understand the Modbus protocol, which I expect is the case, then you should spend some time studying how it works, and then you should be able to understand your specific issues and be able to ask the pertinent questions.

I suggest that you read my guide, "Modbus general description" that is a sticky post at the top of this forum. This will give you a good grounding in the Modbus protocol.

Hello Martin;

When communicating over the Modbus protocol, it is standard to use Modbus addresses for the data you wand to read or write, and then transate that to the addresses of the PLC you re using.

https://en.wikipedia.org/wiki/Modbus#Coil,_discrete_input,_input_register,_holding_register_numbers_and_addresses

Modbus addresses look something like this (taken from the Wikipedia article above); these are older Modicon register numbers, Modbus was originally written for Modicon CPUs in 1979.

• coil numbers span from000001 to065536,
• discrete input numbers span from100001 to165536,
• input register numbers span from300001 to365536,
• holding register numbers span from400001 to465536.

To exchange these register values you must use so-called "Modbus Functions"; see the attached screenshot.

For obvious reason, you cannot use Modicon addresses and functions inside a Siemens S7 program directly. Each S7 family that enables Modbus exchanges has to translate to and from these addresses to read and write the data they want to maniipulate. So reading for an I address and writing to a Q address on your S7 CPU will have to be "translated".

Read more about Modbus would be a good beginning, as our friend smiffy has suggested his excellent introdoctury pdf document can be found here: 

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

It is an excellent initiation to Modbus exchanges. Then read up on the Modbus functions used in your specific family of S7 CPUs to understand how they manage to translate from Modbus to S7 addressing. 

It will become clear as you become more familiar with both addressing standards.

Hope this helps,

Daniel Chartier