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TTI-7 Iss.02 – 06/11
Status Byte Registers
The Status Byte Register contains summary bits from the Questionable Data, Standard Event and Operation
Registers, and also has a single bit reporting the Service Request function for IEEE.
The Service Request function allows the instrument to request service from the system controller. If the same bit is
set simultaneously in both the Status Byte Register and the Service Request Enable Register, the RQS bit is set in the
Status Byte Register and the SRQ (service request) bus signal is asserted. In response to the Service Request, the
controller will normally be instructed to perform a serial poll sequence on all devices. The instrument returns its
Status Byte with SRQ set when it is polled. The action of reading the Status Byte via the serial poll clears RQS and
negates SRQ.
The following describes the Status Byte Register Messages.
Questionable Data Summary (bit 3)
This bit is set when an enabled Questionable-Data event has occurred. It is cleared by reading the Questionable Data
Event Register or by clearing relevant bits in the Questionable Data Enable Register.
Message Available Summary (bit 4)
This bit indicates that the Output Queue contains response data. It is cleared reading all data from the output queue
or, the output queue being flushed by a Query Error or, by the *CLS command.
This bit is particularly useful for synchronising data transfers from the instrument. The application programme can
poll the bit to detect when a measurement is complete and data is available. Alternatively, for IEEE, the
corresponding enable bit in the Service Request Enable Register can be set so that a Service Request (SRQ) interrupt
will occur.
Standard Event Summary (bit 5)
This bit Indicates that an enabled Standard Event has occurred. It is cleared either by reading the Standard Event
Status Register, by clearing relevant bits in the Standard Event Enable Register, or by the *CLS common command.
Request for Service/Master Summary Status (bit 6)
The meaning of this bit depends on how the Status Byte Register is read.
RQS
This bit forms part of the IEEE-488.1 Service Request function and is valid only during Serial Poll. It indicates that the
instrument is requesting service. It is set when an enabled bit in the Status Byte Register becomes set and cleared by
a Serial Poll. Assuming no ‘new reason for service’ occurs, the bit will be set for one Serial Poll and clear for
subsequent Polls. RQS is also cleared if the reason for service disappears, for instance when a status register is read.
MSS
When the Status Byte Register is read with *STB? this bit indicates that an enabled bit in the Status Byte Summary
Register is set. The bit is cleared when all summary-messages are clear.
Operation Summary Message (bit 7)
This bit is set when an enabled Operation event has occurred. It is cleared by reading the Operation Event Register
or by clearing the relevant bits in the Operation Enable Register.
The following commands are used to access these registers:
*STB?
Queries the Status Byte Summary Register. The instrument returns a decimal value which corresponds to the binary-
weighted sum of all bits set in the register.
