Keysight MP4300 Series - Error and Output Queues; Status Diagram

Error and Output Queues
The Error Queue is a first-in, first-out (FIFO) data register that stores numerical and textual description
of an error or event. Error messages are stored until they are read with SYSTem:ERRor? If the queue
overflows, the last error/event in the queue is replaced with error -350,"Queue overflow".
The Output Queue is a first-in, first-out (FIFO) data register that stores messages until the controller
reads them. Whenever the queue holds messages, it sets the MAV bit (4) of the Status Byte register.
Status Diagram
5 SCPI Programming Reference
152 Keysight MP4300 Series Operating and Service Guide