Multiple and mixed modbus tcp equipment using MB_CLIENT (S7-1517)

Dear all,

I'm trying to figure out what's the best and correct strategy to implement multi-server modbus tcp connections (with MB_CLIENT), I'm using a S7-1517 PLC and SCL language. The problem is I have 6 mdb tcp equipment, and they are different from each other. Register blocks I want to read are not close to each other (e.g.40001-40010 and 40200-40206). Let's say there is not a pattern between those equipment.

Should I use one MB_CLIENT instance call and for each equipment (IP) dynamically change the REQ, memory address and lenght, maintaining the same ID (CONN_OUC) for a given equipment, then dynamically change the IP_V4 address, memory address, lenght and ID, REQ, calling the same MB_CLIENT instance for a second equipment and so on (closing and opening through DISCONNECT),

OR

should I use multiple MB_CLIENT instances, one for each request e.g. if I have 20 requests to cover all equipment, I will instance 20 MB_CLIENT blocks and implement coordination (REQ, DISCONNECT, change IP_V4, memory address, LEN, ID...) for each block?

Best Regards,

Diego.

Hi Diego,

From my point of view, I think it would be easier and also less errors could appear after implementation, using the second approach you mentioned.

Hope it helps.

Br,

Cocolino

Hi Cocolino,

thank you for you help. I'll try the second method.

Try to understand the best way to read non-contiguous registers in a given IP and then change to another block with different IP address.

For the same IP and non-contiguous registers, do I need to use multiple blocks? And for these different non-contiguous registers calls for the same equipment, do I need to change the ID?

DiegoMarques

Hi Cocolino,

thank you for you help. I'll try the second method.

Try to understand the best way to read non-contiguous registers in a given IP and then change to another block with different IP address.

For the same IP and non-contiguous registers, do I need to use multiple blocks? And for these different non-contiguous registers calls for the same equipment, do I need to change the ID?

 In that case, to my knowledge, you can do it both ways also. Dynamically using only one block and change the MB_DATA_ADDR, MB_DATA_LEN and MB_DATA_PTR or using multiple blocks, each one with the correct parameters.

Regarding connection ID, in my opinion, each MB_CLIENT must have a unique ID

https://support.industry.siemens.com/cs/ww/en/view/109755202/100612672139

Hope it helps.

Br,

Cocolino

After some simulations I got it working!

I used plcsim advanced and modbus slave to achieve this modbus tcp comm running with no error, but only could simulate one slave at a time in windows (is there a way to simulate multiple slaves with distinct IP/tcp addresses?). I believe they'll run ok in parallell.

The code runs for 4 slaves and I used 4 mb_client blocks, one for each slave (1-Ingersoll Rand Compressor, 1SpiraxSarco Steam Gen. and 2 Chillers).

Each block has a fixed ID and IPv4 cfg and receives a new set of mdb reg address and lenght each 2s to cover all my needed requests.

Maybe the code can be optimized, I spent some lines trying to avoid comm errors (observed traces) but it's running fine when simulated, next step is field commissioning.

Thank you for your support, any tips are welcome!

 I forgot the traces image attachment

DiegoMarques

After some simulations I got it working!

I used plcsim advanced and modbus slave to achieve this modbus tcp comm running with no error, but only could simulate one slave at a time in windows (is there a way to simulate multiple slaves with distinct IP/tcp addresses?). I believe they'll run ok in parallell.

The code runs for 4 slaves and I used 4 mb_client blocks, one for each slave (1-Ingersoll Rand Compressor, 1SpiraxSarco Steam Gen. and 2 Chillers).

Each block has a fixed ID and IPv4 cfg and receives a new set of mdb reg address and lenght each 2s to cover all my needed requests.

Maybe the code can be optimized, I spent some lines trying to avoid comm errors (observed traces) but it's running fine when simulated, next step is field commissioning.

Thank you for your support, any tips are welcome!

Hi Diego,

Thanks for your feedback,good to hear that communication works good.

Br,

Cocolino

Hello,

Just for extra info, note that each MB_Client block will create a TCP connection to your device.

Most modules that are a modbus server have limited (some only one...) sockets free to allow TCP connections. That is the reason why I use a combination of both, I use one MB_CLIENT block for each remote device, where I put the read data in a buffer DB and copy it to the corresponding location. By using a buffer I change the read location and lenght for each poll.

I still wonder why Siemens didn't made a block where you can specify multiple ranges of blocks to read and write data, as with modbus TCP and holding registers, you are limited te read 125 values a one time/poll ....

Kind regards,

Coldrestart.