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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 Questi onable-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 by
reading all data from the output queue or, the output queue being flushed by a
Query Error or, by the *CLS command. This works for IEEE only - the bit is
always 0 for RS-232.
This bit is particularly useful for synchr onising 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. This
works for IEEE only - the bit is always 0 for RS-232.
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 Operati on event has occurred. It is cleared by
reading the Operation Event Register or by clearing the relevant bits in the
Operation Enable Register.
