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Samba - Modbus RTU connection to many slaves

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Dear all, 

I'm programming an application on a  SM43-J-R20 PLC+HMI. I've added a  V100-17-RS4X card, in order to use RS485.

I've succesfully implemented Modbus RTU communication with a Frequency Drive. The thing is, I need to communicate with 11 of them.

The ladder logic so far uses the Modbus function blocks, as can be seen on the Modbus webinars on Unitronics page.

That is, I SET a variable (ex:MB0) , which is used for enabling the "function 06 Modbus"  block (preset holding register).  At the output of the block, I RESET that same variable, ensuring the block gets executed.  At the same time I reset MB0, I SET a new variable (MB1), which in turn is used for enabling the second Modbus block. This creates a sort of "continuous broadcast" to the drivers (which, If they don't receive a command in a certain time, trigger a failure). When the last block gets executed, the sequence restarts,

The question is, when I try to send a Stop command to one driver (for example), it takes an awful amount of time to execute the order (30 seconds or so).

I believe I may have a conflict between some of this blocks being executed at the same time. Is there a tutorial or an example where I can see how to succesfully implement Modbus communication with several slaves at the same time?

Thanks for the support.

Kind regards, 


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Several slaves at the same time is not posible using Modbus RTU using RS485 network.  You must add a short time between Modbus read/write blocks in order to allow each slave to process and respond back and also to allow PLC to process its buffer.

I´ve used delay timer as short as 50 ms

Keep in mind that, if any slave does not respond immediatly, PLC will wait as much time as you set time-out before polling the next slave. This can be add more time than you expect between whole cycle.

My suggestion: if you really need stop a device quicker, use digital outputs instead of Modbus commands.


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