I would try to use the .NET library that is at URL http://www.nmodbus.com/
but I'm
new to mono and mono-develop so :

How can I do to install it on mono-develop?

Thanks to all =)

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I don't know if it is correct but it will be sufficient use the command
shell:

gacutil  NameOfDLL.dll

to install it ??

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The library NModbus works perfectly on Mono. ( http://www.nmodbus.com/ )
I'm using Mono on UBUNTU 8.04 and then:

Develop Mono 1.0
Mono JIT compiler 1.2.6 (mono-V)


The tests I performed were used to establish a serial communication (and
everything works perfectly) but also try short on TCP.
The library is easy to use and at the URL some example code :
http://groups.google.com/group/NModbus-discuss/browse_thread/thread/a0c5e4c1b4362bd9

Thanks to all =)