I am trying to use Float interger to read modbus data, but i have no idea how it works, I was trying Network Analogue Input but it was not possible due to the data range, It looks like this will work But i have no concept of this and data conversion, Im using a RS422/485 Convertor made by Waveshare, i am not able to read any data, the point im looking to read is point 2 which is voltage the paperwork suggest 2 = volts, when i look one site suggests i should take the number and -1 which would make it 1 or add a 1 which would make it 3 neither works, the manufacturer suggests using a function code 3 or a 4, which i assume makes it for +1 3003, i have no idea.. i have tried 2 then 3 then 1 nothing adding it or making it 3002 then 3003 also nothing..any help or a steer in the right direction

Hello Buttsy26,

I don't understand why you open a new topic for the same problem as in this topic:

modbus comms with Logo

Hello Buttsy;

When Siemens provides a Modbus driver it traies to adapt to the Modbus specifications (designed originally foir a Modicon PLC) on its own hardware.

Modbus registers occupy 16 bits, and are used to transfer word-size information, basically integers in the range 0 to 65535 (unsigned values) or -32768 to 32767 (signed values)  respectively.

Since floating-point (REAL) numbers occupy 32 bits (and are formatted very strictly, according to IEEE 754), they occupy 2 seperate but contiguous registers in a Modbus transmission. If your Modbus mapping states that Volts starts at offset address 2, then you should read address 2 and 3, and in your Step 7 program you have to Move them both to a 32-but REAL register so it can be deciphered. as a floating point value.

Unless your Modbus mapping give very different registers for Function 3 and Function 4 registers, very oftwen it does not matter which one you devide to work with, the important part is the offet you are selecting.

Belopw is an example of IEEE 754 conversion, the folating point value you want to read is 240.0 Volts, in the last 2 lines you will find the binary and hexadecival  32-bit values that will be actually sent over your 2 Modbus registers.

Hope this helps,

Daniel Chartier

Edit; I just read your exchanges wth ixo65, and found out that the Logo! was involved in your project. However, Logo! CPUs cannot handle floating point values internally. Look at this link to see about a possible alternative, or better yet, upgrade to a S7-1200 PLC:

https://support.industry.siemens.com/cs/ca/en/view/109741041/89393485963

D.C.

You must manually create a connection between LOGO! and the Modbus device.
LOGO! must be the client, and the Modbus device must be the server.
Then, you must make the entries in the connection table as shown in the screenshot.

so when i added the F/I the Modbus connection was automatically built, and i cant see how i can do this manually, as you note the connectins are oposite modbus is the client and logo is the server, i have deleted the modbus and made a new one and the connection gives me the same pre-configured options

Thanks for the detail, i think moving to S7-1200 would be a good idea but im pretty commited so far, maybe i should look at a better KwH meter or a better modbus convertor, i do appreciate your assistance, i appreciate these are help pages and designed as such, im not an expert hence having to ask. Your response is more than i would expect and for that thanks very much.

The response is much better than most.

Hello ixo65, 

i had no replies to my point in Modbus Comms with Logo so i opened another question, 

I have to ask…

Is it agaisnt the rules to ask what for many experts is a silly question? when clearly i have no idea

Is it agaisnt the rules to ask another question to try and solve the problem? 

Strange as it seems im trying to solve this puzzle. i would like to understand how to make it work?

I appologise if this is something you would rather not help with, feel free to ignore my questions.

Thanks

Buttsy26

Is it agaisnt the rules to ask what for many experts is a silly question? when clearly i have no idea

Is it agaisnt the rules to ask another question to try and solve the problem? 

Don't mind much, somebody here tries to be moderator [...] sometimes. We all have our better and worse days.

 Hello Buttsy26,

to provide a structured forum, I have closed the initial topic with a reference to this topic. This way we don't have the possibility of two parallel discussions and provide the entire thread to users stumbeling across the initial topic. 

Buttsy26

Hello ixo65, 

i had no replies to my point in Modbus Comms with Logo so i opened another question, 

I have to ask…

Is it agaisnt the rules to ask what for many experts is a silly question? when clearly i have no idea

Is it agaisnt the rules to ask another question to try and solve the problem? 

Strange as it seems im trying to solve this puzzle. i would like to understand how to make it work?

I appologise if this is something you would rather not help with, feel free to ignore my questions.

Thanks