LabVIEW
can I broadcast to modbus slave address 0
Obviously on a broadcast the devices can't all answer at the same time so it makes sense that they do not answer at all with any message.
I have however never come across an application that would have needed Modbus broadcasts so I think it is a bit of an esoteric feature. I also can't really remember having read about this possibility in the official Modbus documentation. But that document is anyhow VERY brief on the server address use, so it may not be authoritative on that matter. But it could as well be a custom extension of the protocol by some manufacturer. Another manufacturer may allow for a valid server address of 0 too. My guess is that there is some device that used broadcast on Modbus TCP using a network broadcast address and some zealous engineer somehow wanted to port that to Modbus Serial in some way.
In light of this I doubt it would be a good idea to have NI change the Modbus library to anything else as it is now, unless you can point out a document on modbus.org that states that broadcast to address 0 is an officially specified feature.
You still can create your own copy of the Modbus library where you disable the execution of the Read ADU Packet.vi function when sending a command to the server address (Unit ID) 0.
Just deleting the error is a quick and dirty fix that only works for testing. Each attempt to read data that is not available results in a timeout on the VISA Read node. So everytime you send out data like that to address 0, your VI is delaying execution for the duration of the timeout that is set for the session. That will really slow down your communication loop a lot if you do that more than once.
Instead, if your device is meant to not send anything when addressed with server address 0, you should modify your function to not expect anything to be sent too.
It definitely sounds like the EBM engineers modified the Modbus specification with some features of their own. Something engineers tend to like to do all the time. Take a standard and make it just that tiny little bit better, but of course it means it is then not standard compliant anymore.
And what prevents you to modify the VI library accordingly for this? You can file a bug report with NI, it will be added to the bug report database but it is a big question when they will review this library and possibly make a change. You can wait for that of course but by the time this happens your project is long finished already.
