connection status

[Francisco Ares]

Hi, All.

Is there any means to check for the connection status before any read or write operation? Or is it enough to try read or write and check the result of the operation, and based on that, consider the connection status?

And if the connection is lost, be the program a client or a server, is the re-connection procedure identical as the first connection, I mean, should I delete the context or just keep it and try connecting again?

Thanks!

Francisco

[Julien Blanc]

On 03/03/2015 13:43, Francisco Ares wrote:
> Hi, All.
>
> Is there any means to check for the connection status before any read
> or write operation? Or is it enough to try read or write and check the
> result of the operation, and based on that, consider the connection
> status?

You can’t check without trying to read / write (at least in TCP, for RTU
i’m not sure). So yes, you need to check the return value.

>
> And if the connection is lost, be the program a client or a server, is
> the re-connection procedure identical as the first connection, I mean,
> should I delete the context or just keep it and try connecting again?

Delete the context and create a new one. That’s inexpensive, and will
guarantee you a clean start (i remember having had issues with reusing a
context).

Regards,

Julien Blanc

[Francisco Ares]

2015-03-04 5:48 GMT-03:00 Julien Blanc <julien...@nmc-company.fr>:

On 03/03/2015 13:43, Francisco Ares wrote:

Hi, All.

Is there any means to check for the connection status before any read or write operation? Or is it enough to try read or write and check the result of the operation, and based on that, consider the connection status?

You can’t check without trying to read / write (at least in TCP, for RTU i’m not sure). So yes, you need to check the return value.

Sorry, I forgot to tell I'm using TCP mode. Just for completeness, Modbus TCP mode is a protocol over another protocol, and still there are the TCP/IP logical layers down to the ethernet physical one (that's why I asked), but as far as we are using just the top protocol, I understand and it makes perfect sense when you say that there is no other way of checking connection status;  but for RTU mode, it's Modbus directly over a physical layer, so the answer, as far as I can understand, is the same, there is also no other way to check for connection status but trying to read or write something.

And if the connection is lost, be the program a client or a server, is the re-connection procedure identical as the first connection, I mean, should I delete the context or just keep it and try connecting again?

Delete the context and create a new one. That’s inexpensive, and will guarantee you a clean start (i remember having had issues with reusing a context).

Thanks for the tip. my program was written to keep the context and I got some weirdness, maybe that's what you are talking about. After the program was modified to delete the context on read or write operation error (and also closing the connection and trying for a new one, of course), now it works as expected.

 

Regards,

Julien Blanc

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Thank you again, and Best Regards.

Francisco

[Stéphane Raimbault]

You don't need to delete the context, you can just close the connection and open it again (modbus_close/connect).

See http://libmodbus.org/docs/v3.1.2/modbus_set_error_recovery.html for information about the ways to handle connection errors in libmodbus.

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[Francisco Ares]

Thank you, gonna try it out immediately.

Best regards,

Francisco