Hello, first of all, thank you very much for
 the patience you have with me and with this
 modbus question.
I have tried to change the way of
 communicating.
 I have tried now that the plc is 
master and write in my external device, 
which I have programmed as a slave ... but
 the error continues, as I mentioned before. 
 As you say I have reviewed the port and I have 
placed the port that Tia Portal hardware
 indicates to me ..... in fact I have not
 written it but I have clicked on the
 possibilities of the menu and in this 
menu I have chosen the port that corresponds
 to the PTP module that I have connected.
 I have checked that this port is the same 
one that comes in the constants table of
 the system as in the DB of the 
modbus _comm_load and also in the hardware
 configuration of Tia Portal that is the port
corresponding to the physical communication
 module that I have connected to the CPU
 as you can see in the PDF the three 
coincide is the port 263.. The cpu error 
disappears if I stop calling mdbus_master 
from ob1. Therefore it seems that the error
 is within this block

so I am very confused and I do not know how .

Thanks for the help you are giving me

It's false. That wants to say that itis not

being executed, I suppose ...  The block is called only once because it is in OB1 but it is called by bool "first scan". There is something

more to tell you; when I "erase" or stop calling

the modbus master module in OB1, the error in the

CPU disappears ..... .Thank you

Sorry...It is true. I appreciate it, but when

I click on the link you sent me:

https://support.industry.siemens.com/tf/ww/en/posts/problem-modbus-rtu-et200sp-as-eslave/How%20do%20you%20evaluate%20the%20status%20of%20system%20blocks% 20in% 20STEP% 207% 20 (TIA% 20Portal)?

 I get this: 

Thanks

Kaulquappe

Sorry, I didn't insert the correct link. Second try:

How do you evaluate the status of system blocks in STEP 7 (TIA Portal)?

Hello

I have read several posts from people who have encountered this same problem. In my case, as in them, the parameters of parity, bit stop, speed ... etc are fine. I have also checked the connection cable and exchanged TX RX, I have checked the parameter Line_Pre ... etc I am learning to use the previous library (SaveStatus ") and I run into a problem. As I understand in the siemens support page, the link takes me to a website where the status status parameter is analyzed, like a 4-byte array. However the FB "SaveStatus", contained in the library that is downloaded from this page has an array of 10 DWord. I guess they are different things. I am analyzing the elements of the DB array corresponding to the "SaveStatus" of the master, but I do not know what part of the communication corresponds to each element of the array to be able to locate where the error is affecting. In the help of Tia Portal does not speak to me about this FB "SaveSTatus". Is there a page that explains what each element of the 10 in this array represents?

Thank you

Hello everyone

Finally I have been able to solve (more or less; not completely) the issue of communication, but it works. I share my solution here in case it is useful for someone:

The ModbusCommLoad block is called in the first scan cycle with the parameterization of the serial communication. The ModbusMaster is called in a cyclic ob. To write several analog values ​​from my PLC ("master") to the other device (slave), I use an auxiliary program, so that, in communication, I ignore errors 16 # 81E2 16 # 81E7. Both errors appear at the beginning of each communication. I have not been able to disappear, I have changed the connection cable, I have isolated the "GND" well, checked the connection parameters ... etc. The values ​​are sent from a DB with access not optimized in the form of words, one by one through an auxiliary program that "sweeps" all the records to be sent, cyclically, to send only 1 word at a time. When I tried to send more, to me personally, problems have arisen.

In this way, finally, my modbus connection works (with some limitations, especially in time, because it sends the records cyclically, one by one), but it works for me and the slave reads well what my PLC sends.

Thanks to all who have helped me

Regards