Hi everybody.

I am working on a project with PCS7 V8.0 software where I have to use a Modbus Master Module RTU RS-485 Protocol.  The model of the module is 6ES7341-1CH02-0AE0. I decided to make a test with an Allen Bradley PLC Micrologix 1100. At the moment of writing data, everything is OK, both coils and registers (I only need a F8 SND block).

My problem begins when I want to read data from the slave (reading data) because according to the manual I need a group of two blocks(F8 SND and F7 RCV) to reach a reading command.

Attached to my post, I draw a picture that describes this issue. At the upper edge of the pricture "Problem 1A" there are a pair of blocks whose function is to request data from the slave 6 so that I am able to read data from 0001 to 0100 according to the configuration from DB1500. The information from the slave must be recived in DB2013 according to the configuration from RCV block.

At the lowest edge of the picture "Problem 1A" there are a pair of blocks whose function is to request data from the slave 6 so that I am able to read data from 40001 to 40050 according to the configuration from DB1501. The information form the slave must be received in DB2017 according to the configuration from RCV block.

When I send a request (2) to the second group of blocks, the data from the Allen Bradley PLC are transfered to the block DB2017 and all the information is shown in every word into this block, but when I send a request (1) to the first group of blocks, the data from the Allen Bradley PLC are tranfered to the block DB2013 and "misteriously" all the data located in DB2013 are also transfered to the block DB2017 (A), as a consequence all the information is replaced (INCORRECT X) without a logic explanation because this pair of blocks work independenlty and there are no relationship between them, no matter if I change the number of block or the type of function, the behavior is the same.

The configuration is correct and the dongle download is also OK as well as the cable connections.

Please, I need your help to solve this. See the other pictures attached for more details

Thank you for your response.

In order to ask for a request in my current test, I send the bit REQ in every pair of block separately and manually so that I am able to see the beheviour.

This error happens when I ask for a reading request in the first pair of blocks.

Do you believe I need a firmware update?

Hi,

Below is reply for you query,

You have to call FB 7 & FB 8 only once, Calling Twice is not allowed for same CP-341. However you can use different Send DB as well as Receive DB with same FB 7 & & FB 8. But to keep it simple use one Send DB with different multiple receive DB.

This makes life easier in sense or reading holding, Input or coils in modbus. 

Then You need to make a polling sequence program where you can use Send DB and Receive DB for calling. Generate a bit which will call FB 7 & and FB8 in a same bit and in addition to that same bit changes the values in send DB as per requirement of Modbus query like slave ID, FC no register start address etc. Make sure you are moving the same Receive DB number for which you are sending query in Send DB.

*Hit like if your problem is resolved by this