What could be the reason for sporadic slave failure on the AS-I bus?
Check the actual length of the AS-I line. A typical error with AS interface is exceeding the maximum permissible line length of 100m with using an extender or repeater. If the maximum length is exceeded, this might lead to sporadic or permanent slave failure on the AS-I bus.
When checking the line length, don't forget to take into account the length of the AS-I cables that you have installed in the switchgear cubicles. Very often these cable lengths are forgotten.