modbus yet again!


Thread Starter

Mario de Sousa

Gilles Allard wrote:

> Receiving a Modbus RTU message is not easier.
> The inter-character timer is not implemented in hardware. So it
> must be implemented in the driver (retriggered on every
> interrupt).
> You should also disable the FIFO on the UART.

It's not that I'm shying away from writing a device driver (it would not be my first...), but I would guess that most users would prefer to
have it implemented on a user process, rather than having to mess around with installing device drivers...

> I do not think this protocol will be easy to implement on a
> general-purpose OS.

Yes, I have come to that same conclusion myself.

Greg, I believe you support Modbus on the PuffinSCADA? Can you give us an idea of how you went about working around the issues we have been
discussing? Or better yet, would it be possible to publish your modbus code under the GPL? It doesn't make much sense to duplicate our efforts

I am working with Phil's code as a base, and it is working fine under normal circumstances, but I'd like to make sure it would also work if an
aborted frame ever came it's way.

Another question:
Although I was trying to avoid it (I am using Phil's code that already implements it), I have ended up reading most of the Modbus spec. BUT, it is not yet clear to me whether the spec allows more than one master on the same bus. I would guess not, since it does not cover the possibility of clashes of two or more masters writing at the same time to the bus, but I did not find it stated explicitly anywhere. Am I
missing something here?

Thanks in advance,


Mario J. R. de Sousa
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The box said it requires Windows 95 or better, so I installed Linux

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