Protocol Bytes with dynamic element

[Mamut1975]

Congrats to the nice project. Very cool HW. I tried to analyse the protcol for door open at my BJ room station using my scope and an UART decoder. I could read all bytes. 2x 13 bytes are visible. I wonder because in byte 9 is a dynamic section. Byte 13 seems a CRC byte with unknown calc approach.

Every time when I open the door a different random number show up.

Any idea what this is? Any idea how the CRC is calculated?

I got for example the following 13 bytes

55.FF.03.0D.40.01.10.01.B1.51.9F.05.1F 55.FF.03.0D.40.01.10.01.45.51.9F.05.CC 55.FF.03.0D.40.01.10.01.2F.51.9F.05.AC