HP 8508A Operating & Service Manual page 60

Operation
Queues
_. The Output Queue
There are two queves in the status reporting structure of the Vector Voltmeter.
The Output Queue is a "first-in, first-out" (FIFO) queue. It stores output messages until they are read from the
instrument. The availability of data is summarized in the MAV bit of the Status Byte. The Output Queue is read by
addressing the instrument to talk and then handshaking the bytes. It can be cleared by the Reset Command, the
Device Clear Command (IEEE488.1) or by power on.
The Vector Voltmeter also has an Error Queue, which operates as a FIFO and stores error messages until they are
_ read from the Vector Voltmeter. The Error Queue is summarised in the error bits (bits 2 to 5, inclusive) of the
Standard Event Status Register. It is read by the SYSTem:ERRor ? command and can be cleared by using the
Clear Status command (*CLS). When the Error Queue is empty, the string "0, NO ERROR" is returned.
Event Registers
Event Registers capture changes which occur within a device. Each bit in an event register corresponds to some
device condition. Bits become TRUE when some pre-defined device condition transition occurs. The event register
bits are latched so that once they become TRUE they cannot be cleared except by the user. There are two means of
clearing an event register. Reading a register will clear it and the Clear Status command (*CLS) will clear all
event registers.
The Vector Voltmeter uses two kinds of transition filter for setting these event bits TRUE:
1. Positive Transition. The event becomes TRUE when its condition makes a FALSE to TRUE transition.
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