How to read Inputs, Holding Registers and write Coils to a Modbus TCP Slave device

Dear all,

I would like to ask some questions regards Modbus TCP Master Communication. I'm using S7-400H PLC with CP443, and I'm trying to read Inputs, Holding Registers from a slave device, and also write Coils to the same slave device. In the program, I'm using Siemens library block MB_REDCL to establish the communication.

At the inputs of MB_REDCL, there are a few inputs like 'DATA_TYPE', 'START_ADDRESS', 'LENGTH' and 'WRITE_READ' for me to set the function and addresses of Modbus Communication. But the inputs here are only for one function at one time, and there are length limits for the data to be read or written.

I understand that with the parameter DB, I can set up to 8 different function and address areas. Then I tried using parameter DB at the input 'db_param', but it was not working. The Modbus Communication only follow the setting of inputs ('DATA_TYPE', 'START_ADDRESS', 'LENGTH' and 'WRITE_READ') of MB_REDCL. 

So if the MB_REDCL only used the inputs 'DATA_TYPE', 'START_ADDRESS', 'LENGTH' and 'WRITE_READ' to exchange data, does it mean the values of 'data_type_x', 'start_x' and 'end_x' are not useful?

And for my requirement, since I need to read Inputs, Holding Registers from the Slave device and write Coils to the Slave device, then I need to call MB_REDCL 3 times for each operation. For this case, the same connection ID and CP Hardware address are used in the parameter DB. Then I call each MB_REDCL sequentially.

I triggered 'ENQ' input of the first MB_REDCL manually, then use the 'DONE' output of first MB_REDCL to the input 'ENQ' of the second MB_REDCL. Same as the third MB_REDCL. All 3 MB_REDCL FBs used same inputs except 'db_param', 'DATA_TYPE', 'START_ADDRESS', 'LENGTH' and 'WRITE_READ'.

Unfortunately, this was not working. Only the first MB_REDCL was done successfully. 

Can anybody give any comments on this issue?

 Dear Sir,

Thank you for the reply.

For the parameter datablock configuration, are there any limits for the data range? For example,  for the 1st data type, I set it as 2, which is Inputs. Then I put 'W#16#0' for the 'start_1', and 'W#16#BB8' for 'end_1', which has the data range of 3000. 

But as I saw from the Modbus manual, input 'LENGTH' of MB_REDCL has a range limit from '1 to 2000' for Inputs. So does this range limit also apply to the range which defined in the parameter datablock (3000 in my previous example)?

Thanks.

Wüstenräuber

Hi,

in the parameter datablock the modbus registers are defined, which the modbus block should know and in which datablock the modbus registers are stored. It is not a job list. It is only the definition, that the modbus block can read or write in the correct datablock.
Have a look at the manual, it is described in it, also with a picture.

Modbus/TCP is a sequencial communication. You can only start the next telegram, when the response from the previous telegram was received. 
It is not allowed to insert the modbus block with the same ID multiple times. You have 1 modbus block instance with 1 ID and this is called in each CPU cycle 1 time. (You can't use the same connection ID in different modbus block instances.)
For different telegram types (Holding Registers, Inputs etc.) you must call the modbus block and change the input parameter DATA_TYPE, START_ADDRESS etc.

Here you can download a library, where a job list block is included: https://support.industry.siemens.com/cs/ww/en/view/62830463

Wüstenräuber