AstroDMx Technical Help for Linux

ZWO USB2 Camera Issues

ZWO USB2 cameras can be problematic under Linux. This is due to the fact that the camera’s default firmware uses a USB maxPacket size of 1024 bytes instead of the standard 512 bytes. Previously, in order to get these cameras to work, one had to either install the compatible firmware (available from the ZWO website) or patch the Linux Kernel. The compatible firmware fixed this problem by using a maxPacket size of 512 bytes. Patching the Kernel fixed this problem by stopping Linux from blocking USB2 devices which had an invalid maxPacket size.

If your Linux distribution is blocking invalid maxPacket sizes then the camera will be seen by AstroDMx but it will never enter streaming mode. The preview display will permanently say, “Initialising, please wait…”

Although the compatibility firmware fixed problems associated with ZWO USB2 cameras, it introduced a new problem which meant that the camera could not be used in 16-bit mode for long exposures. Therefore, it was not a perfect fix for this problem.

Fix Option 1

Current Linux kernel versions no longer block USB2 devices with an invalid maxPacket size and so, these cameras should now work out-of-the-box and with the standard firmware. The author of this website is not sure when this change happened, but it appears to be somewhere between kernel versions 4.14.0 and 4.15.0. Therefore, if you are using a kernel version greater than 4.15 your ZWO USB2 camera should work without any modifications unless the following is true.

Some modern computers do not have a EHCI USB controller. On such computers, the xHCI controller handles all ports, including USB2. Unfortunately, the USB2 ZWO cameras do not seem to be compatible with the xHCI controller unless the compatible firmware is installed. If your computer does not have a EHCI USB controller then your only option will be to use the compatible firmware which is available from the ZWO website.

Fix Option 2 – Patching the Kernel (Historic)

As mentioned above, modern kernels do not require patching. However, if your distribution does not offer a newer kernel version or, for whatever reason, you want to use an older kernel version then you can patch the kernel. Note, patching the kernel will only work if your computer has an EHCI controller. As there are so many different distributions, I will not explain how to actually compile the kernel. For this information, please consult your distribution’s documentation.

The following instructions will show you how to modify the kernel source code to stop it blocking invalid maxPacket sizes. Start by downloading the kernel sources by following your distribution’s kernel compilation instructions. Once you have the kernel sources downloaded, navigate to the root of your kernel source and edit the following file.

drivers/usb/core/config.c

Search the config.c file for the following.

static const unsigned short high_speed_maxpacket_maxes

Once you have found the correct block of code, you should see something similar to the following.

static const unsigned short high_speed_maxpacket_maxes[4]
{
    [USB_ENDPOINT_XFER_CONTROL] = 64,

    [USB_ENDPOINT_XFER_ISOC] = 1024,

    /* Bulk should be 512, but some devices use 1024: we will warn below */

    [USB_ENDPOINT_XFER_BULK] = 512,

    [USB_ENDPOINT_XFER_INT] = 1024,
};

Edit the number, 512 at line 9 and change it to 1024. If it is already 1024 then your kernel is already patched correctly. Once edited, save the source file and continue with building the kernel by using your distribution’s instructions.