PLC:  ......... S71200, CPU1214, FW4.1
SOFTWARE: ..... TIA Portal V13 SP1 Update 8
PROTOCOL: ..... Modbus RTU (RS-485)

 Hi everyone.

I am busy constructing coding for the S7-1200 to link with the Modbus protocol to a 3rd party device. The older version instructions { MODBUS } works, but the newer version instructions { MODBUS (RTU) } does not. Below picture shows where those are found.

Simply put - Has anyone worked with the instructions under { MODBUS (RTU) } and got it working correctly?

THINGS I HAVE DONE
I took a piece of working code and changed it to the new version Modbus instructions.

Already with the first scan call of Modbus_Comm_Load, an error code is registered. Below is the error code and its meaning.

By going deeper into the provided help the following information could be sourced..


These are internal (STAT) to the instruction Modbus_Comm_Load and can be linked to via the DB that is linked to it. The following is an example of this:

The original code was adapted to have the following structure - note the first MOVE instruction. (The code below is to show the structure only, not the actual code.)

By using the above MOVE instruction the original alarm did not come up anymore when the communication was initialized. BUT a new alarm appeared.

It is that at this point that I stopped further development - hoping that there is someone that has gone through this R&D already and willing to share. I know that the previous version worked, but hoped the new version could be utilized instead.

Do you know how to use the instructions for { MODBUS (RTU) } instead of { MODBUS }?

Cheers.
w

Hello my friend,

I also looking for that (mean will try, test it) if will have free time, maybe today, will post results. Good luck...

Duris_I

Hello my friend,

I also looking for that (mean will try, test it) if will have free time, maybe today, will post results. Good luck...

Hi there friend Duris - it is very good to hear from you.
Thank you for your promising note.

I have some more information that might help.

I have quickly worked through some Forum posts and FAQs. A somewhat helpful post was:
[Modbus_master freezing in Busy]

There is an example that a member did for an ET200SP (CPU1512). The communication module has some more adjustable hardware settings available than what I have on the S7-1200. See below.

I am assuming that the instructions contained in the { Modbus (RTU) } folder were meant as a universal solution but there are stuff missing for the S7-1200.

Best regards.
w

Hello William,

made quick test and looks like success! Look to my pictures, S7-1212 v4.1.1 , CM1241 RS-232, on PC runing MB_Slave application from Simply Modbus - perfect SW for test. Imported that Modbus RTU library instructions v3.0, only Master tested and working for me. I was prepared only memory for REQ signals to drive my logic, again use static value of my CP HW (habbit ) and use instance DB statuses, no problem. After run Modbus_Comm_Load REQ - status OK, then run via^%M0.1 Modbus_Master instruction and receive 4 words from register address as example. Working to me without problems, that Mode is 0 as Help stated with full duplex RS-232. In OB1 run that FC where are that Modbus_RTU instructions, no OB100 yet. Via Watch table seting up that %M0.0 and %M0.1 signals. For receive using DB3 Master_RCV with 10 WORDS defined, but only 4 WORDs test, is enough for test. Hope it helps you, maybe I'll be available to you again with other infos

Update: that Mode - 0 is located in instance DB of Modbus_Comm_Load_DB, in my case as I post, is 0 -> RS-232. Don't have RS-422/485 module. Tested TIA v13 SP1 upd2

Duris_I

Update: that Mode - 0 is located in instance DB of Modbus_Comm_Load_DB, in my case as I post, is 0 -> RS-232. Don't have RS-422/485 module. Tested TIA v13 SP1 upd2

Thank you Duris!

So now we know that the communication works for RS232. So the probability is high that the MODE state that I have found is the only problem..... maybe.

I will have a look at your code.

I will also test the Modbus_Comm_Load again on my RS485 setup.

We'll chat again on this subject.
w

Duris_I

Update: that Mode - 0 is located in instance DB of Modbus_Comm_Load_DB, in my case as I post, is 0 -> RS-232. Don't have RS-422/485 module. Tested TIA v13 SP1 upd2

Duris_I,

Your notes and photos was a great help. I could get the communications running on the "new" version for Modbus communication.

Potential issues was identified. There are "settings" that is not properly handled by the Modbus_Comm_Load instruction when working with S7-1200. An online view is shown of the data contained in the DB linked to the instruction..

The internal MODE parameter needs to be set to 4 when using RS485. In the case of RS232, the default of MODE = 0 is not a problem. My case needs RS485.

The change in my own program looks like..

The added Network 5 made the communications work properly. The MODE setting is essential for RS485. A communication speed up to 115.2k baud  - with read and write cycles - could be achieved without too much issue.

BUT I still need to get 24 networked devices to work together. There is bound to be timing issues where timing settings for the communication will play a bigger roll. Some of the above red marked settings may still play a roll.

Some other notes...
For my application the PLC needs to manipulate "coils" (bit) and "registers" (numbers). The coils are handled by addresses with the format 0xxx, while registers are addresses 4xxx.

When addressing a coil with a write cycle -
  With the older Modbus instructions,
                     16 consecutive bits was manipulated.
  With the newer Modbus instructions,
                     1 bit is manipulated through the word.

-----------------

I hope that someone from Siemens also observes the issue identified in this post. Or maybe someone having trouble with the instruction gets help in this way.

Duris_I - thank you for your support.
w