A quick Google search turns up a reference [qtforum.org] of somebody using QextSerialPort for a RS-485 device. So, that should work, I guess. That, of course, does not implement iec 62056-21 on top of that yet. Does that get you started? Note that I don’t have personal experience with any of this :-)

Thanks Andre,
I already know QextSerialPort and I have no problem in communicating via rs-485. My problem is to know iec 62056-21 .
Reading some manuals it seems that iec 62056-21 is a standard protocol but I can’t find some documentation about.
I only found this [satec-global.com] IEC 62056-21.pdf but it’s for a specific device and I actually can’t try that protocol for my device.

Ah, ok. I think the details of iec 62056-21 are really outside the scope of Qt itself, and are off topic on this forum. Then again, perhaps that is why you posted this in The Lounge?

Andre wrote:

perhaps that is why you posted this in The Lounge?

Yes, I posted here because of this. :-)