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Modbus TCP, check forwarded data


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47 minutes ago, ORSO2001 said:

Hi All,

I am using the Unistream PLC as a slave connected by Modbus TCP...exist a way to know/check the forwarded data, by the Master, byte to byte?

 

thanks

Not sure why you want to ( error checking, byte manipulation, etc)?

Read this and see if it answers any of your questions.  

 

https://www.rtaautomation.com/technologies/modbus-tcpip/

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hi dierkens

thanks for answer....maybe my explanation was not clear...my PLC works as a slave...i create some registers (read, write) for a master (that is not mine)...who has created the master program is saying me that the comunications is not working...but if i use a program test to check my registers all works fine...i need to know what they are passing me.

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51 minutes ago, ORSO2001 said:

hi dierkens

thanks for answer....maybe my explanation was not clear...my PLC works as a slave...i create some registers (read, write) for a master (that is not mine)...who has created the master program is saying me that the communications is not working...but if i use a program test to check my registers all works fine...i need to know what they are passing me.

You should use something like a MODSCAN to test the reading/writing of registers/coils on both ends.

Although you indicate that your PLC works as a slave, you must mean that they are sending the MODBUS register information to your PLC.

You can use WIRESHARK to get down to the packet level of information; but that level of detail typically isn't needed.

  1. Can you ping the MASTER device?
  2. Can the SLAVE ping you?
  3. Have you checked the OFFSET to make sure that both devices are using the same array start [Zero (0) based versus One (1) based register assignments].  Start with something easy like coils or a lower register.  

Just use some basic MODBUS troubleshooting techniques.

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hi dierkens,

thanks again for your answer...now I have 5 minute of "relax" to try to explain better what I have.

to be clear ...when the phisical installation was started my PLC was ready, program done, test field simulation etc, to be connected with the master...obviously I wrote a data exchange document with all explanation...also a step by step example... was who had to program the master side that was not ready...and develloped the program at customer site!

at the time I have connect the PLC with the PC where I created an easy program in C# to test...to show at the end customer that all work fine...and it was.  after a few the second supplier told me that they were having problem because they never used Modbus protocol (but was the agreement from the beginning!?)...I explained again what they had to do...after a few they told me "now is ok"....great I thoug...but now they come back telling me that sometimes the command is not catch...in my PLC I have a "debug page" where I can see the amount of the "success" and "failed" about the TCP side...the fail are always "0" and the "success" increased every time.

in their PLC they don't manage the exception or the data send error or don't check "my" registers about alarms or wrongs commands....I know because the customer user don't found these when the system seems not working...and they (second supplier) are not telling me how they are creating the data string that are sending...

yes I can use something like a "sniffer" but I was thinking if the buffer of the data entry was available!?

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