HP 81101A Reference Manual page 22

General Programming Aspects
Status Model
Status Byte
The status byte summarizes the information from all other status groups.
The summary bit for the status byte actually appears in bit 6 (RQS) of the
status byte. When RQS is set it generates an SRQ interrupt to the
controller indicating that at least one instrument on the bus requires
attention. You can read the status byte using a serial poll or *STB?
Standard Event Status Group
22
Bit Description
0 Unused, always 0
1 Unused, always 0
2 Unused, always 0
3 QUESTionable Status Summary Bit
4 MAV—Message AVailable in output buffer
5 Standard Event Status summary bit
6 RQS; ReQuest Service
7 OPERation Status summary Bit, unused
Bit Description
0 Operation Complete, set by *OPC
1 Unused, always 0
2 Query Error
3 Device Dependent Error
4 Execution Error
5 Command Error
6 Unused, always 0
7 Power On