checksum error, Modbus RTU

Hi,,, friends,,, I have a new problem with com mod mast function,, I have a checksum error,, but when I use a modbuspool the error disappears,, I check automation builder configuration and is correct but the error continue,, I cant read and write in the slave,, but with modbus salave and modbus pool simulators I can do it, what is my problem,? I am use the.com 1 of the pm554 eth,,,, the plc is new,, the port can have damage,,? or what is the reazon of th checksum error.,,

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dannypat   

asked 27 days ago
Closed



Best Answer

1

If you have a doubt on your COM PORT 1 , Make it as a ONLINE ACCESS. Then try to download the Program through COM 1 using USB to Modbus converter cable.

If you can Attach your Modbus settings screenshot, Slave details here .

vigneshwaran92rce   

answered 26 days ago


 


By dannypat on 7/24/2017 | Like (0) | Report

Hi vignes,.¡¡ thaks,., I chek COM1 and it´s works correctly,.¡¡¡ I changed de NB in COm_mod_mast and I can write in the slave but in the com_mod_mast still have error,, my dude is,. the NB is the number of bytes that send but its only data? or include other byte,. to complete the modbus frame,.?


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Answers

1

If you have a doubt on your COM PORT 1 , Make it as a ONLINE ACCESS. Then try to download the Program through COM 1 using USB to Modbus converter cable.

If you can Attach your Modbus settings screenshot, Slave details here .

vigneshwaran92rce   

answered 26 days ago


 


By dannypat on 7/24/2017 | Like (0) | Report

Hi vignes,.¡¡ thaks,., I chek COM1 and it´s works correctly,.¡¡¡ I changed de NB in COm_mod_mast and I can write in the slave but in the com_mod_mast still have error,, my dude is,. the NB is the number of bytes that send but its only data? or include other byte,. to complete the modbus frame,.?


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0



WRITE_DATA: ARRAY [0..4] OF WORD;

With this configuration the PLC send the data, but in ERNO still have the checkSUm error and error 16383 "Transmission is not possible at the moment. Another instance of the Function Block is already transmitting" why,.?? this is to write I´m try also read with the FCT 3 but not read nothing,.¡¡¡ thanks for your time Vignes.... I´m student and is my first PLC and my first mdbus comunication and I have problems with this,¡¡¡

 modbuscap.JPG

dannypat   

answered 25 days ago


 


By dannypat on 7/24/2017 | Like (0) | Report

The slave is a variator no ABB, It´s supports modbus RTU, and I try to read the same directions that I write with FCT 16 but nothing.
I check the slave configuration, baudrate, direction, timeout and is correct,, the slave parameter that I want write and read are "0000....000A" the variator has more parameter but I only want changed the firts 10


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0

Now your COM port working Fine.

Did you read and write your slaves through MODSCAN (or any other sw)?

If yes , your Modbus register is correct. If no find your slave Modbus register.

As per your comments, Transmission is not possible at the moment. Another instance of the Function Block is already transmitting"


My suggestion is ,

1.Remove your blink block.

2.Instead of blink ,create 1 local variable.

3.Make that high ,test 1 cycle read or write.

4.check your done bit ,error bit.

vigneshwaran92rce   

answered 25 days ago


 


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0

Hi,

In the image posted by you, I observe that the no of Stop bits in PLC COM port configuration is 2. Set this to 2 only if the Slave has been configured with Stop Bits=2. Most slaves have by default 9600 baud rate,8 Data bits, Even parity and 1 Stop bit. In the blink block set 100ms ON time and 1000ms OFF time,since slave response time out is set to 1000ms.

While experimenting with Modbus communication, I had once observed that, the Modbus master software (in my case modscan32) would be able to communicate with a slave irrespective of whether Stop bits in Modscan32 software was set to 1 or 2.

However, this will not be the case with AC500 PLC as a master, as it strictly enforces all rules of serial communication.

Try to initially assign a cycle time of 500 ms for the task which is executing the MODBUS program. The 500ms cycle time is a good starting point for tasks handling Modbus programs.Later on, you may reduce the cycle time based on your observations of any errors arising due to further reduction in the cycle time.

Regards

Rahil

rahilmohammed   

answered 24 days ago


 


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