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Mainboard connection

References

The different printers in the Kobra series have different mainboards. Kobra 3, Kobra 3 Max and Kobra S1 ones are different but use a very similar structure.

Here is a step by step guide on how to do it with a Kobra 3 mainboard without the SW2 switch: Kobra 3 mainboard with missing SW2

Building the cable

The motherboard has USB firmware access using a JST MX / Molex PicoBlade 1.25mm pitch connector. You will need to build a cable to make it usable using a classic USB-A or USB-C connector.

If you want to use a USB-C female connector, you will need to use a USB-A to USB-C cable to connect to your computer. USB-C to USB-C might not work properly.

Follow the wiring shown in the pictures. Beware, the colors don't mean anything here and your cable colors may vary. Here:

  • Red is VCC
  • Black is Data (-)
  • Yellow is Data (+)
  • White is GND
  • USB-C end of the cable
    USB-C end of the cable
  • JST end of the cable
    JST end of the cable

Motherboard preparation

Then you will need to get access to the motherboard. Unscrew the bottom plate of your printer, there are 2 long screws and some clips to access the inside.

Once inside, you will need to disconnect all the wires and cables marked in red in the picture below. They will prevent your computer from recognizing the motherboard properly.

Depending on your printer things will be plugged at different locations. The goal here is to remove any power-hungry devices that would prevent the MB from booting using USB power.

Mainboard with cables to disconnect highligted in a red box

On this Kobra 3 example, the JST / Molex connector is in blue and the serial debug connection is in pink.

Computer connection

At this point, you should be able to plug your JST / Molex connector to the motherboard.

Hold the SW2 switch (in orange in the picture above) or short its pins and plug the USB connector to your computer.

Now check if the USB device is recognized properly by typing lsusb. You should see a “Fuzhou Rockchip” device with the ID 2207:110c listed.

Here are a couple of potential issues and solutions:

  • Retry the connection process multiple times, it can be flaky. Wait between each attempt
  • Don't release the SW2 button too quickly
  • Depending on your USB-C connector, only one side might work
  • Use dmesg -w on your computer to have more information

Note

Some computers will simply refuse to recognize the device. For example, my desktop computer refused no matter what cable or Linux distro I used, but my laptop worked the first time.

Starting recovery mode without SW2

SW2 is used during boot to instruct the MB to start recovery mode. Unfortunately, shorting pins of SW2 is not enough to start recovery.

Some (I feel newer) mainboards have this component missing. Here is a comparison of one mainboard with the SW2 button and another one without.

Comparison of early and late unit of MB around SW2
Comparison of early and late unit of MB around SW2

We can see that late MB units have some missing components around SW2, namely: SW2, D7, R24, C25, C26 and Y2.

The missing R24 component is a 100Ω resistor in line with the SW2 switch and needs to be replaced. Then it will be possible to short SW2 pins to boot in recovery mode.

Warning

Using such big components on small SMD pads is risky as you may easily rip one pad off. I didn’t have smaller one here to demonstrate

An example using a big old 100Ω resistor
An example using a big old 100Ω resistor

Tooling preparation

Now that the connection is working properly, you will need to collect and compile some tools:

Flashing procedure

Initialize the connection

Switch the motherboard in the right mode to interact with the computer:

$ sudo ./xrock maskrom rv1106_ddr_924MHz_v1.15.bin rv1106_usbplug_v1.09.bin --rc4-off
# This command should return directly

Check the connection by reading flash information:

$ sudo ./upgrade_tool rfi
Using config.ini
Flash Info:
    Manufacturer: SAMSUNG,value=00
    Flash Size: 7456MB
    Block Size: 512KB
    Page Size: 2KB
    ECC Bits: 0
    Access Time: 40
    Flash CS: Flash<0>

Read / Backup your partitions

Dump the partition layout:

$ sudo ./upgrade_tool pl
Using config.ini
Partition Info(parameter):
NO  LBA        Size       Name
01  0x00000000 0x00000040 env
02  0x00000040 0x00000400 idblock
03  0x00000440 0x00000400 uboot_a
04  0x00000840 0x00000400 uboot_b
05  0x00000c40 0x00000200 misc
06  0x00000e40 0x00008000 boot_a
07  0x00008e40 0x00008000 boot_b
08  0x00010e40 0x00018000 system_a
09  0x00028e40 0x00018000 system_b
10  0x00040e40 0x00020000 oem_a
11  0x00060e40 0x00020000 oem_b
12  0x00080e40 0x00100000 userdata
13  0x00180e40 0x00080000 ac_lib_a
14  0x00200e40 0x00080000 ac_lib_b
15  0x00280e40 0x00020000 ac_app_a
16  0x002a0e40 0x00020000 ac_app_b
17  0x002c0e40 0xffffffff useremain

Dump all partitions:

sudo ./upgrade_tool pl | tee output/partitions.txt
sudo ./upgrade_tool rl 0x00000000 0x00000040 output/env.img
sudo ./upgrade_tool rl 0x00000040 0x00000400 output/idblock.img
sudo ./upgrade_tool rl 0x00000440 0x00000400 output/uboot_a.img
sudo ./upgrade_tool rl 0x00000840 0x00000400 output/uboot_b.img
sudo ./upgrade_tool rl 0x00000c40 0x00000200 output/misc.img
sudo ./upgrade_tool rl 0x00000e40 0x00008000 output/boot_a.img
sudo ./upgrade_tool rl 0x00008e40 0x00008000 output/boot_b.img
sudo ./upgrade_tool rl 0x00010e40 0x00018000 output/system_a.img
sudo ./upgrade_tool rl 0x00028e40 0x00018000 output/system_b.img
sudo ./upgrade_tool rl 0x00040e40 0x00020000 output/oem_a.img
sudo ./upgrade_tool rl 0x00060e40 0x00020000 output/oem_b.img
sudo ./upgrade_tool rl 0x00080e40 0x00100000 output/userdata.img
sudo ./upgrade_tool rl 0x00180e40 0x00080000 output/ac_lib_a.img
sudo ./upgrade_tool rl 0x00200e40 0x00080000 output/ac_lib_b.img
sudo ./upgrade_tool rl 0x00280e40 0x00020000 output/ac_app_a.img
sudo ./upgrade_tool rl 0x002a0e40 0x00020000 output/ac_app_b.img
sudo ./upgrade_tool rl 0x002c0e40 0xffffffff output/useremain.img
# The last one is dumping indefinitely, not sure why yet

Modify the partitions

Expose the dumped partition as a device, for example with userdata:

$ sudo losetup --find --show output/userdata.img
/dev/loop0

The file /dev/loop0 is now mapped to your parition dump. Now we can mount it to a directory in order to access and modify it.

$ mkdir -p _userdata && sudo mount -o rw /dev/loop0 _userdata

At this point, the directory _userdata is your partition and you can modify the content. Everything is directly flushed to the partition dump, so make sure the original dump is backed up properly.

Write / Flash the partition

To write a partition back to the motherboard, reuse the offset from the partition list to write the partition at thr right location, for example for userdata:

$ sudo ./upgrade_tool wl 0x00080e40 0x00100000 output/userdata.img