Bernd> Yes - you must use 1 - there is only one out-endpoint. 0x81 is Bernd> for receiving data and endpoint 0 is the mandandory control Bernd> endpoint. Interrupt Endpoints are not variable in size. Both Bernd> interrupt endpoints are 8 Bytes, so you must read and write Bernd> exact 8 Bytes per transfer - 5 shouldn't work for USB compliant Bernd> devices.

I took your earlier advice and just opened ugen0.1 O_RDWR. I've tried sending 5 and 8 byte strings to it without effect. To make the 8 byte strings, I added spaces to the end of my 5 byte strings. I've also got a proper multitester now to verify operation.

Bernd> Depends on the device's firmware. I wouldn't be surprised if Bernd> the whole device just hangs after receiving bogus data - it Bernd> seems to be broken in many points. But it may block if the Bernd> device has nothing to send. An easy way to check out is using Bernd> tools like usbdevs -v and see if it hangs when accessing this Bernd> device.

usbdevs -v and udesc_dump both operate fine after trying this.