Added feature RTU over TCP

[xabies]

Hi. As you told me in #257 I have made a pull request for you.

I have adapted this patch https://groups.google.com/forum/#!msg/libmodbus/xm8S6MMDhgM/Qd97y9_mhwgJ for the current version of libmodbus.

As I told you I'm a very newbie as C programmer and I am sure that my solution is not the best. If you could take a look to my branch I think you could rewrite a better solution without problems. I think that this feature could be interesting for other people too.

Thanks for your job.

[mhei]

Indeed, this would be a cool feature. And you are right, the implementation is ... straightforward 😄
I'll try to test it next weekend, however, I cannot promise.

I wonder, whether we should to think a little bit more general but simply connecting to a socket and implement a RFC2217 layer. Thus we could also support ethernet to serial gateways which support changing baudrate etc. I know this would require some more work...

[xabies]

This will be great mhei. I simply find a patch for a previous version of modbus and I apply it to the master branch. The code isn't event mine. But I agree, this feature could be interesting for other people too, so I will be waiting for news.

Thanks.

[DaveMDS]

Just a confirmation:
I'm successfully using this patch (on libmodbus 3.1.4) to read a ModbusRTU over TCP device.

RTU over TCP is indeed a poor and non standard protocol, but sadly it is used in the real world. So it would be cool to have this included in libmodbus.

Regards

[v-zhuravlev]

Hello, any news on this? Really looking forward to this feature in the master branch in order to implement it in https://github.com/v-zhuravlev/libzbxmodbus
Thank you in advance!

[myssta]

Please consider my approach (#445). It contains both server, client, test, documentation code. however I assumed that the slave id must be set and considered. Please comment especially whether the slave id should be considered or not. I assumed RTU considers it so RTU over TCP should do it, too.

[alorle]

I think this functionality is essential. It is not the ideal case, but in real life RTU over TCP is widely used as discussed in #257 (comment)
In our use case, it is essential and what limits us the most today.
@stephane could you take a look at it and rate its inclusion. I am available in case help is needed for this implementation.