There are two timing loops in the BLHX firmware.
500us loop = servo parameter update
2ms loop = input/output, amin Xlanguage instruction command set
Registration operates inside the 500us loop. The BLHX responds to a hardware registration input and captures the encoder position within 20us. However, the data is not acted upon until the next 500us interrupt, when the registration move will be processed.
Overall the postion will be captured and the move will be processed within a 1ms time frame.