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Serial-over-USB connectionΒΆ

Specifically to V2+. This can be used for terminal access from the managed server to the PiKVM, or for any other purpose that requires a serial connection. In the last case, you only need to perform step 1 and reboot.

USB limitations

TL;DR: By default, you can add only one additional device to choose from, such as USB Ethernet, or USB Serial, or an extra Mass Storage Drive.

There is a hardware limit on the number of devices that can be emulated at the same time. Each USB device uses so-called endpoints to communicate with the host. Depending on the type, the device consumes a different number of endpoints, while their total number is limited by the capabilities of the chip, for Raspberry Pi it is 8.

It is quite difficult to calculate the number of endpoints used, but in the case of PiKVM, you can focus on the following numbers:

Device Endpoints
Keyboard, mouse 1 for each
Mass Storage Drive 2 for each
USB Ethernet, USB Serial 3 for each

V2 and V3 emulates one mouse by default, V4 emulates two mouses. Thus, V2 and V3 use 4 endpoints, and V4 uses 5 by default.

Creating an axtra Mass Storage Drive consumes additional endpoints, as well as USB Serial and USB Ethernet, so only a limited number of devices can be selected for the final configuration, for example, one USB Ethernet.

If you need something more non-standard, you can disable the regular Mass Storage Drive and the additional mouse (on V4) to free up some extra endpoints.

The kvmd-otg service is responsible for setting up USB emulation. If the endpoint limit is exceeded, the service will not be able to start and no emulated USB device will work.

In the log it looks something like this:

# journalctl -u kvmd-otg
kvmd-otg[382]: kvmd.apps.otg                     INFO --- ===== Preparing complete =====
kvmd-otg[382]: kvmd.apps.otg                     INFO --- Enabling the gadget ...
kvmd-otg[382]: kvmd.apps.otg                     INFO --- WRITE --- /sys/kernel/config/usb_gadget/kvmd/UDC
kvmd-otg[382]: OSError: [Errno 524] Unknown error 524
kvmd-otg[382]: During handling of the above exception, another exception occurred:
kvmd-otg[382]: Traceback (most recent call last):
kvmd-otg[382]:   File "/usr/bin/kvmd-otg", line 9, in <module>
kvmd-otg[382]:     main()
kvmd-otg[382]:   File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/", line 348, in main
kvmd-otg[382]:     options.cmd(config)
kvmd-otg[382]:   File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/", line 278, in _cmd_start
kvmd-otg[382]:     _write(join(gadget_path, "UDC"), udc)
kvmd-otg[382]:   File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/", line 83, in _write
kvmd-otg[382]:     with open(path, "w") as file:
kvmd-otg[382]: OSError: [Errno 524] Unknown error 524
systemd[1]: kvmd-otg.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: kvmd-otg.service: Failed with result 'exit-code'.
systemd[1]: Failed to start PiKVM - OTG setup.

In this case, you need to disable some of the previously enabled devices and restart PiKVM.

  1. Edit /etc/kvmd/override.yaml and add these lines:

                enabled: true
  2. Run the following command:

    # echo ttyGS0 >> /etc/securetty
  3. Create the directory /etc/systemd/system/getty@ttyGS0.service.d and add a file file named override.conf into it. Afterwards edit the file and copy this into it:

  4. Run these comands:

    # systemctl enable getty@ttyGS0.service
    # reboot
  5. Once PiKVM is rebooted you will have access to a virtual serial port on the server that the USB is connected to. Use mingetty, screen, putty, or something like this to access the kvm from the server. The port is called /dev/ttyAMA0.