Which mbed? Can you disable flow control? Without hardware flow control enabled the PC shouldn't make any assumptions about how the DSR line will act.

LPC1768 with quite old libraries (build 26) (bit afraid to update). It used to be default behavior. Disabling flow control is not an option (talking about measurement equipment that I need to be sure of is up and running).

It doesn't matter what you set your PC port to. There is no hardware flow control. Take a look at the schematic if you want https://developer.mbed.org/media/uploads/chris/mbed-005.1.pdf there is no hardware flow control on that port.

The flow control lines you were seeing were imaginary ones generated by the USB controller and the driver on the PC in order to keep legacy software happy. All the flow control will be happening in software in the USB stack, that will be happening regardless of what you've set the port to on your PC.

I know these schematics, but i see different behavior after upgrading the flash version. The DSR 'line/bit' is no longer detected by the PC (where it was previously).

Btw updated from default tp version 141212.

So the question is, what has changed?

I have no idea what's changed, that's why this isn't an answer.

But your question wasn't what has changed. Originally you asked if there was a way to manually control the line implying that you didn't want to know what had changed, you wanted to know how to work around the changes.

Disabling hardware flow control should fix it and shouldn't have any impact on the final behavior. If you know the schematic then you should know that your statement that "Disabling flow control is not an option" is meaningless since there is no hardware flow control on the physical serial port.

Thanks and I agree about the question (but your answer trigger me).

With the same application I also support a slightly different equipment that has a serial port and can be used directly or with a USB serial converter.

As i also need to detect the PC serial port the mBed is currently using, I used DSR as a way to determine if something is alive on a serial port (without assuming it's an USB based one).

Btw http://www.nxp.com/documents/user_manual/UM10360.pdf is probably a better document to look at. Although I could not find direct evidence it shows serial over USB is more complex then just two USB signal wires.

Have a look at section 11.6.2 for example.

The USB over serial is more complex. As I said previously the USB will implement flow control no matter what the settings.

But the schematic shows that the processor that implements the USB serial port is then connected to the main processor via a physical UART. That port is the one I'm talking about. It's two wires, Rx and Tx, no flow control.