S7-1200 Modbus RTU Questions

Bullzi · Jan 22, 2021

Hi Everyone,
First Project with Siemens and I need to communicate via RS-485 to a Tank Level Hub using Modbus RTU. I will be using the CB 1241 485 card.
I am just kind of getting started but have a couple questions:
1) How does the Siemens PLC handle REAL values? I will end up reading 2 registers but how do I combine the 2 registers into a REAL in the TIA?
2) How do I set up the word and byte order in the software? I haven't seen where to do that in the MB_Comm_Load or MB_Master blocks. It is possible that I have missed something in the DB and to tell you the truth my head is swimming trying to wrap my head around just how different Siemens is vs. Allen Bradley, GE and other PLC's I have worked with in the past. So please forgive me if I miss something obvious

drbitboy · Jan 22, 2021

https://support.industry.siemens.co...at-in-the-step-7-(tia-portal)-?dti=0&lc=en-AF

?

e.g.

L D[AR2P#0.0] · Jan 22, 2021

Classic case for a function, two word inputs, one real return value. Code it in SCL as SCL can be exported as text and is easily transferred.

cyoung120 · Jan 22, 2021

Here is a link to a YouTube video that I found very helpful. I also attached a sample program that I think I downloaded from the videos comment section.

https://www.youtube.com/watch?v=mlTOuWWVbiM

https://www.youtube.com/watch?v=q4g1uy2U0g4&t=232s

Hopefully this helps. I would be willing to share some of the stuff I have done with modbus RTU. Just can't post on an open forum.

Good Luck

cyoung120 · Jan 22, 2021

Sorry, file is too big to attach. PM me your email if you would like and I will email to you

Bullzi · Jan 25, 2021

Thanks cyoung,
I tried to download the file that came with the second video but it is version 13 and I cant open it in V15.1. Guess it would need to be updated to V13 SP1 before I can open it.

L D[AR2P#0.0] · Jan 25, 2021

The file in the second video was TIA V14 - anyway, attached as V15.1

cyoung120 · Jan 25, 2021

L D[AR2 said:
The file in the second video was TIA V14 - anyway, attached as V15.1

Thanks for converting it. I was going to do it in the morning when I got to work.

Bullzi · Jan 26, 2021

Thanks again LD,
I am starting to get ahold of this now and I think I was making it much harder in my mind than it really is.

I have a question about DB's. In the example program the author created a DB's called MBCONFIG(DB2) and MBLOAD(DB1) which makes sense to me. But then in the System Blocks - Program Resources there are 2 more DB's MB_COMM_LOAD_DB(DB4) and MB_MASTER_DB(DB5). I understand that DB1&2 are user created DB's and 4&5 are Instance DB's created when the MB_COMM_LOAD and MB_MASTER Instructions were used. What I noticed is it appears that the info in DB1 could be sourced from DB4 and the info in DB2 could come from DB5. So my question is what is the advantage of creating your own global DB's to store info that is already stored in an instance DB?

Thanks for the advice!!

L D[AR2P#0.0] · Jan 26, 2021

As you point out, the data is duplicated in the global DB and you could eliminate some of the content. To see what you can eliminate, delete all the parameters to the function block - anything in red has to have a parameter so hence the need for a global DB (or M flag etc.) The non-red parameters can be accessed from the instance DB, as I've recoded in the screen shot below.

The code was probably copied from a Siemens example and that is the coding method that they use.

Bullzi · Jan 27, 2021

Thank you that makes total sense now!!

Bullzi · Feb 9, 2021

Well I am now trying to test the Modbus Communications to a my computer using Modbus Slave program. The issue I am having is I get a 818C error in the MB_Master block. I am not getting the TX/RX light to light up on the CB1241. When I look up that error it says that I am using an invalid PTR. It all looks right to me but I am sure I am missing something.

If someone could take a look at the screen shots and let me know what I am doing wrong.

Lare · Feb 9, 2021

Have you inited succesfully you CB card for Modbus RTU with Modbus_Comm_Load block?

Also your DB maybe should be non optimized (classic DB) with DB3.DBD addresses. You screenshots looks like that addresses are missing.

p.s It should works also with non optimized, but you can try example P#M100.0 WORD 32 if error is still same.

Bullzi · Feb 9, 2021

I found the issue. I forgot to disable the Optimized Block Access in the DB I was sending the data too. I just unchecked it and everything is working. Sorry for the false alarm.

S7-1200 Modbus RTU Questions

Bullzi

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drbitboy

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L D[AR2P#0.0]

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cyoung120

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Bullzi

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L D[AR2P#0.0]

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