XH-HK4401 4-port HDMI USB KVM Switch

This KVM is sold under many names, and comes in two versions. The only way these two versions differ is that one has one of its USB ports replaced with a PS/2 port. The identifying feature is that they come with a small external control unit with 4 buttons. This controller is connected to the main KVM via a micro USB cable, however this is NOT as USB connection.

!!! warning Audio was not tested, it is assumed to be non-functional

Connections

Connect the USB-A cable from the Raspberry Pi OTG port to to any of the USB ports on the XH-HK4401 switch. All 3/4 USB ports work exactly the same, internally they are just connected to a USB HUB.
Connect the HDMI out from the XH-HK4401 switch to the Raspberry Pi CSI-2 to HDMI input.
Connect host USB and HDMI cables from the XH-HK4401 switch to the machines to be managed per the switch instructions.
Finally see below for details about connecting to the control micro USB port. This it not a normal USB micro port.

!!! warning There is a limitation in the underlying PiKVM software related to plugging video cables from a host which is already powered and connected to a monitor to a Raspberry Pi HDMI-CSI bridge. These limitations apply equally when using the XH-HK4401 KVM switch. If video is not present in PiKVM, try keeping all host machines off and connecting them directly to the XH-HK4401 switch before powering the hosts on.

RS-232 control cable

The control unit communicates to the KVM using the RS-232 protocol (at 5v) not USB, and one of the following solutions must be used.

An inverter circuit

For this you will need:

2x 2n7000 MOSFETs
2x 10K resistors
1x USB Micro connector, or sacrificial micro USB cable
Optional: USB UART adapter

You can connect this either via a USB UART adapter, or directly to the Raspberry Pi: GND -> Pin 6, TX -> Pin 8, RX -> Pin 10. On the v3 PiKVM hat you will need to disable the UART jumpers to use the on-board UART.

!!! note Please search online for USB pinouts to ensure you connect it properly.

Inverting USB UART adapter

Some USB UART adapters have the rare feature to invert the logic level of the RX/TX signals. For example the FTDI FT232 can be configured via the FTDI configuration GUI to do this. With such an adapter, the circuit above is not required. All you need is to connect it to a micro-USB connector.

In order to invert the RX/TX signals, you can use ft_prog and set the following settings:

!!! warning These options will only be available on UART adapters with genuine FTDI chips. There are a lot of cheap fakes on the market that either lack this option, or will prevent you from changing their settings. To avoid getting a fake ensure you always purchase from a reputable store and brand (Adafruit, Sparkfun, etc.), Amazon is not a reputable store.

Adding UI elements to control the KVM switch

The UI can be updated to add buttons to switch between KVM inputs and indicators for which input is currently selected. The instructions below will make these available in the PiKVM UI after clicking the "GPIO" menu button in the KVM view.

SSH into PiKVM
Enable read-write mode on the sd card via rw

Edit the /etc/kvmd/override.yaml file and include the following. Note the assumption that you are using a USB UART present on /dev/ttyUSB0:

kvmd:
    gpio:
        drivers:
            hk:
                type: xh_hk4401
                device: /dev/ttyUSB0
        scheme:
            ch0_led:
                driver: hk
                pin: 0
                mode: input
            ch1_led:
                driver: hk
                pin: 1
                mode: input
            ch2_led:
                driver: hk
                pin: 2
                mode: input
            ch3_led:
                driver: hk
                pin: 3
                mode: input
            ch0_button:
                driver: hk
                pin: 0
                mode: output
                switch: false
            ch1_button:
                driver: hk
                pin: 1
                mode: output
                switch: false
            ch2_button:
                driver: hk
                pin: 2
                mode: output
                switch: false
            ch3_button:
                driver: hk
                pin: 3
                mode: output
                switch: false
        view:
            table:
                - ["#Input 1", ch0_led, ch0_button]
                - ["#Input 2", ch1_led, ch1_button]
                - ["#Input 3", ch2_led, ch2_button]
                - ["#Input 4", ch3_led, ch3_button]

Return to read-only mode for the sd card via ro
Restart the kvmd service: systemctl restart kvmd

Switching between hosts in the UI

To switch between hosts, enter the KVM UI and click the "GPIO" menu. You should see 4 inputs, one of which will have a green circle indicating it is currently selected. Click the other inputs to change the selected host.

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