Maintaining 'Headless' Linux embedded systems: USB Keyboard Emulator and USB HDMI Capture

USB Keyboard Emulator. Top, RS485 USB Dongle and bottom: RS485 USB Dongle reprogrammed as USB HID Keyboard

We have a few hundred embedded Linux systems installed by now, and we have always maintained them remotely where possible, by connecting them to a GSM/LTE/4G modem, and then logging in from the office. Where this is not possibly we plugged in via CAT5 copper LAN.

Since we do not use the monitor or keyboard in these cases we often do not install them. The Linux CPU itself might not be easily accessible; for example, it might be mounted on top of a lift.

There are some types of failure where Linux would not start sufficiently to run its network stack or even boot up. This means we could not log in at all. In these cases we would need to attach a monitor and keyboard.

I got tired of having to lug around a laptop, monitor, full-sized keyboard in addition to a rucksack filled with tools and spares. The laptop has an LCD screen as well as a keyboard. Why should I have to carry two of those?

The laptop screen can be connected to the embedded system using a USB HDMI Capture dongle. I got a no-name Chinese make to start with.

USB Video Capture Device HDMI Capture Card USB 2.0 1080P HD Game Video HDMI Capture Card

It cost only RM58 but did not come with an HDMI cable. And it pretty much worked right out of the box and came up as /dev/video1. I simply treated it like a TV dongle:

mplayer -cache 128 -tv driver=v4l2:width=1080:height=720:outfmt=i420  -vo xv tv://

The picture isn't great, it wobbles somewhat and I had to crank up the gamma setting on the laptop LCD, but hey, it sure beats lugging a monitor around.

Pretty soon the keyboard started getting on my nerves. After all it is a much simpler device, why shouldn't I be able to get a USB dongle, plug one end into my laptop and the other into the embedded Linux CPU? It was surprisingly hard to find, in fact I could not find one at all.

We make a lot of USB to RS485 dongles, used mostly for Modbus interfacing. I could just take two of them, connect them back-to-back via their RS-485 ports and plug one into my laptop. It would come up as /dev/ttyACM0, a serial port. At a pinch I could use minicom to act as a crude keyboard.

The other USB to RS485 dongle would be a little more tricky. I would need to reprogram it as a USB HID Keyboard so that the embedded Linux system (indeed the BIOS before that) thinks it is a keyboard.

Luckily I had based design of the USB to RS485 dongle very closely on Microchip's Low Pin Count Development System using the PIC18F14K50 CPU. And Microchip had provided sample code for it including code for USB HID Keyboard.

I immediately reprogrammed my dongle as a keyboard and plugged it in. And sure enough it came up as:

[Wed Jan 31 19:48:48 2018] usb 1-4.5.4: New USB device found, idVendor=04d8, idPr
oduct=0055
[Wed Jan 31 19:48:48 2018] usb 1-4.5.4: New USB device strings: Mfr=1, Product=2,
 SerialNumber=0
[Wed Jan 31 19:48:48 2018] usb 1-4.5.4: Product: Keyboard Demo
[Wed Jan 31 19:48:48 2018] usb 1-4.5.4: Manufacturer: Microchip Technology Inc.
[Wed Jan 31 19:48:48 2018] input: Microchip Technology Inc. Keyboard Demo as /dev
ices/pci0000:00/0000:00:14.0/usb1/1-4/1-4.5/1-4.5.4/1-4.5.4:1.0/0003:04D8:0055.00
08/input/input22
[Wed Jan 31 19:48:48 2018] hid-generic 0003:04D8:0055.0008: input,hidraw5: USB HI
D v1.11 Keyboard [Microchip Technology Inc. Keyboard Demo] on usb-0000:00:14.0-4.
5.4/input0

Next I copied over the RS232 code from the same Microchip sample set, 'USB Device - CDC - Serial Emulator', and that worked too.

Lastly I needed to match the minicom keystrokes (where were RS485 ASCII) to the keyboard scan code. The former can be found here and the latter here. It took just two days to prototype, a USB keyboard emulator.

[Updated 2021-03-09]

Despite initial misgivings about using minicom, it is possible to work around this by assigning the less-commonly used ASCII codes, like SI (Shift In), decimal value 15, which minicom can generate as Ctrl-O. Similarly for commonly-used BIOS keys like F2, F5, F6, F9 and F10.

What I need is an improved USB HID keyboard program to send the correct scan codeswhen triggered by a minicom-generated ASCII code, so watch this space.

Happy Trails.