XHC WHB04B development?

I had/have very similar issues as you mention earlier today Hakan. The after the pendant disconnected once, it kept doing so every 20 s or so, just like for you. And you see the LIBUSB_ERROR_NO_DEVICE because the dongle has disconnected. However, if you again start LinuxCNC (maybe need to try a couple of times to get the timing right), the pendant will resume working just fine and the disconnects stop for the time being.

As I have said, I believe the reason for the disconnects is that the dongle is unhappy with the current situation and resets itself. And "unhappy with the current situation" is most likely because the host has not been polling its interrupt endpoint for some time (maybe half a second or so).

I attach here a logic analyser screenshot, capture file as well as wireshark capture from a "forced" disconnect event. For this capture, I had the dongle disconnected, LinuxCNC (sim) running a program and in the background waiting for the pendant. I then started all captures, connected the pendant, pressed the Step button (and maybe some others) on the pendant and jogged the wheel. This caused the Usb::setupAsyncTransfer assert failed.

In the screenshot you can see that from approximately 2950 ms (corresponding to wheel jogging), there are no packets until ~3600 ms (except for SOF, which I have turned off on purpose). And at 3750 ms, there is a reset on the USB bus, corresponding to the dongle "disconnecting" and the assert failed. This reset is as far as I'm concerned generated by the dongle itself, because it was unhappy with the conditions.
 

If I zoom in a little bit on the disconnect/reset event, that becomes more clear. Here I have turned the SOF packets back on, and we see that they are sent every 1 ms as expected. And then suddenly the D+ signal drops to zero, indicating a bus reset (either from the device or the host). And I see no reason why the host should generate a reset in this situation, so I say its generated by the dongle. This is consistent with repeated disconnects every 20 s already discussed.
 

The .sr file can be opened with PulseView and the .pcapng with Wireshark, in case anybody wants to have a look.

At least for me the pendant is anything but solid. And I do think the dongle firmware is mainly to blame. That being said, it could very well be that different drivers/hardware could mask some of the issues I experience. If only I knew what causes (all) the disconnects, I'm sure we could figure out some ways to fix it.

If you just have the dongle and pendant attached and switched on do you get resets every 20 sec?
I mean, without linuxcnc.

I took the dongle and pendant to a bit newer N150 PC. Plugged it in. Ok, was discovered.
Switched on the pendant and it started with the 20 sec disconnects/reconnects.
Switched off the pendant with the button and it stopped. There is a clear relation.
It can be related to polling, I guess.
Searched for a simple program to regularly poll a usb device.
Best I found was "usbreset" but it didn't change anything.

Interesting that you MRx says it is stable. Advice was to test with USB 2.0 and I have looked
and it isn't easy to find a PCI-E USB 2.0 board I can get in a few days. My motherboard
Gigabyte Z170N-WIFI does have an internal header for USB 2.0/1.1 and I should get a cable
and test I guess.

I tested now and get same behavior as you. This was some small news for me. I thought it only started reconnecting if it had already been in use already by LinuxCNC.

The test was like:
1. Pendant off, plug dongle into computer. No disconnects for 5 minutes.
2. Turn pendant on. Disconnects start with around 20 s cycle.
3. Turn pendant off. It looks like dongle stays disconnected. Maybe just bad/good luck with the timing.

I already made some first steps for a new driver. The initial target of that was to have reliable polling. I already abandoned it and started thinking more about the firmware. But maybe I should give it another shot. I'll try to have a look tomorrow if I can clean it up just slightly and post it here. It's simple enough to play around with and it should be able to provide stable polling.

I'm interested to try it out.

Can there be different versions of the usb dongle firmware?
Like newer devices don't have the reconnect issue?