Modbus RS485/422 und Broadcast

Hello Thorsten,

As far as I know RS485 standard defines maximum of 32 devices on the same wires. For more you need a repeater. Otherwise you are out of specification scope and risk reliability. It might work in your case (especially if total cable length is much, much less then 1200m), but it is not recommended at all. Online translation was not clear about having everything in one or more segments, so forgive me if I misunderstood your configuration.

If I remember well, I have implemented broadcast in both RTU and ASCII MODBUS slave implementations. The slave simply executes MODBUS valid command but does not reply at all. I will have to check this for confirmation. If you are in a hurry, then you can check it yourself if you send command to change some data in slave with address 0, and then try to read it (use real address instead of 0) and see if it was changed or not. The problem with broadcast is that you can not know if all your slaves were live at the moment of sending the message, accepted the message, and then executed the command from the message. There isn't any confirmation at all. If confirmation is important to you, you will have to implement it on your own, since broadcast confirmation is not supported by the standard. Maybe you can put some autoincrement byte as last one in the long message(s), and then check each slave if they all have correct value of this byte. One long broadcast message and many short individual checks should be considerably shorter then many long individual messages.