USB_CDC DTR bad docu?

[kreasteve]

CircuitPython version

8.0.4 on Pico/Tiny2040

Code/REPL

import usb_cdc
usb = usb_cdc.data

if usb.in_waiting:
    print(usb.readline())

Behavior

No data is received if DTR is not active.

The note in https://docs.circuitpython.org/en/latest/shared-bindings/usb_cdc/index.html#usb_cdc.Serial.connected seems to relate to the whole thing, not only on usb_cdc.Serial.connected.

Maybe you want to update the documentation.

I tried and searched a bunch of things but I can't get it working without DTR.

Description

I'm using hterm and c# and have this issue since the versions newer as 7.3.3

Additional information

And related to

You cannot create an instance of [usb_cdc.Serial](https://docs.circuitpython.org/en/latest/shared-bindings/usb_cdc/index.html#usb_cdc.Serial). 
The available instances are in the usb_cdc.serials tuple

in https://docs.circuitpython.org/en/latest/shared-bindings/usb_cdc/index.html#usb_cdc.Serial

I never found an usb_cdc.serials tuple. I get the Serial object from usb_cdc.data

[dhalbert]

The note in the doc says (https://docs.circuitpython.org/en/latest/shared-bindings/usb_cdc/index.html#usb_cdc.Serial.connected):

The host is considered to be connected if it is asserting DTR (Data Terminal Ready). Most terminal programs and pyserial assert DTR when opening a serial connection. However, the C# SerialPort API does not. You must set SerialPort.DtrEnable.

This is an idiosyncrasy of the C# API. So if your C# code does not set SerialPort.DtrEnable, .connected with not be true, and the behavior will be exactly you said:

No data is received if DTR is not active.

The tuple reference is wrong -- that's from an older beta version of the API, sorry.

[dhalbert]

If you are saying that the Note about DTR should also say that no data is delivered if DTR is not asserted, that would be a fine additin.

[kreasteve]

I have a project where circuitpython is communicating with a c# program. I do not use usb_cdc.Serial.connected and it don't work with the normal read methods without DTR. This is confusing me, because it works until 7.3.3.

[Neradoc]

The change comes from #7100 that was originally for an issue where using the serial channels too early would cause a crash, so it checks for a connection. The result is that we get a situation where there's something in the buffer, but it can't be read if nothing is connected.

I commented on that here: https://github.com/adafruit/circuitpython/issues/7456#issuecomment-1383495476 (I intended to open an issue for a discussion on that, but I wanted to do some tests first with different configurations and didn't come back to it)