Hello

I am working on a small logic application that requires 16 devices to communicate (Modbus TCP) with one PLC (working as Modbus Server: reading inputs, writing outputs, handling logic).

Devices have inputs and outputs so I must use two connections resources to read bits and write bits. S71200 has limited open user communication (which Modbus is), and it is max 8 connections. So in my case only 4 peripherial devices.(2x MB_CLIENT with 0 and 1 MB_MODE)

Can I extend max number of connections somehow?

Is there any PLC that will allow more Modbus connections?

@vulpes:
There is some example written up in the S7-1200 System Manual. The relevant piece is now attached.

Good luck.

@Kaulquappe:
I am happy to have an expert answer. Please help me understand this point. I get confused on when to use CLIENT and when use SERVER. (_{Sorry for hijacking the topic.})

Assume there are remote stations (variable speed drive or remote IO) that the PLC needs to link to to read/write data from/to. How is the TCP-CLIENT instruction used to pole the different Modbus addresses?

Is it required to use multiple resources to do read and write?

Thank you.

 Hello and thank You for anwser.

Kaulquappe

Hello vulpes,
in your case will the PLC act as Modbus client.

Yes, my mistake: Clinet (or master). Correct.

Kaulquappe

With the input DISCONNECT of MB_CLIENT you can terminate the connection and establish it to the next server. In this case the servers are requested sequentially and the maximum number of connections won't be the problem.

OK, so token method.... how long do You think I need to establish connection? Unfortunately UDP is not allowed. I can make code to base on time, DONE and ERROR bits, but also need to make exception handling if the connection is not engaged several times in a row.

Is it a question od miliseconds?

Kaulquappe

You need only one connection per device even you use read and write requests. 

This I dont know how to do..... I can select MB_MODE as 0 1 or 2 and writing excludes reading. Unless You were reffering to token method...

Regards,