USB input device limit of 5


I have a BeagleBone Black and want to attach a number of numpads via USB. Unfortunately it is not possible to connect more than 5 USB numpads. It seems like a kernel or device driver limitation since the bus topology does not matter. The sixth numpad always produces the following (/var/log/syslog):

input: USB Keyboard as /devices/ocp.3/47400000.usb/

input: failed to attach handler kbd to device input40, error: -16

hid-generic 0003:1241:1503.0028: input,hidraw11: USB HID v1.10 Device [ USB Keyboard] on

I’ve connected a powered 4 port Hub with another unpowered 4 port hub but have tried several other topologies.

Maybe someone knows what the problem is or knows what the obscure error number 16 stands for.
Thanks in advance.