Agilent Technologies 8712ET Programmer's Manual page 58

Bits in the Status Byte register are set to 1 under the following
conditions:
Device Status Summary
(bit 2) is set to 1 when one or more enabled bits in the Device
Status event register are set to 1.
Questionable Status Summary
(bit 3) is set to 1 when one or more enabled bits in the
Questionable Status event register are set to 1.
Message Available
(bit 4) is set to 1 when the output queue contains a response
message.
Standard Event Status Summary
(bit 5) is set to 1 when one or more enabled bits in the Standard
Event Status event register are set to 1.
Master Summary Status
(bit 6, when read by *STB) is set to 1 when one or more enabled
bits in the Status Byte register are set to 1.
Request Service
(bit 6, when read by serial poll) is set to 1 by the service request
process (see
Operational Status Summary
(bit 7) is set to 1 when one or more enabled bits in the
Operational Status event register are set to 1.
Programmer's Guide
"Generating a Service Request" on page
Using Status Registers
The Analyzer's Status Register Sets
5-8).
5-13