Appendix B: Status model
Model 2110 5½ Digit Multimeter Reference Manual
B-6
2110-901-01 Rev. C/August 2013
Status structure
Using serial polling and SRQ
Any enabled event summary bit that goes from 0 to 1 will set bit B6 and generate a SRQ (service
request). In your test program, you can periodically read the Status Byte to check if an SRQ has
occurred, and what caused it. If an SRQ occurs, the program can, for example, branch to an
appropriate subroutine that will service the request.
Typically, SRQs are managed by the serial poll sequence of the instrument. If an SRQ does not
occur, bit B6 of the Status Byte Register remains cleared, and the program proceeds normally after
the serial poll is performed. If an SRQ does occur, bit B6 of the Status Byte Register will set, and the
program can branch to a service subroutine when the SRQ is detected by the serial poll.
B6 stays set until all Status Byte summary bits are reset.
The serial poll automatically resets B6 of the Status Byte Register. This allows subsequent serial polls
to monitor bit B6 for an SRQ occurrence generated by other event types. After a serial poll, the same
event can cause another SRQ, even if the event register that caused the first SRQ has not been
cleared.
The Master Summary Status Bit (MSS) is set when an enabled summary bit of the Status Byte
Register is set. This bit (B6) may also be interpreted as a Request Service (RQS) bit. Depending on
how it is used, Bit B6 of the Status Byte Register is either the Request for Service (RQS) bit or the
Master Summary Status (MSS) bit:
•
When using the serial poll sequence to obtain the status byte (serial poll byte), B6 is the RQS bit.
•
When using the *STB command to read the Status Byte Register, B6 is the MSS (Message
Summary Status) bit.
The serial poll, although automatically resetting the RQS bit, does not clear MSS. The MSS remains
set until all Status Byte summary bits are reset.
To use an SRQ:
Configure the instrument to generate an SRQ when the ESB bit is set. Use the Status Byte Enable
Register (SRE) to select the low-level SRQ signal set by summary bits. When the status byte B6 is
set, the instrument automatically sends the SRQ interrupt message to the bus controller, which may
poll the instruments on the bus to identify which one requested service. Querying the Status Byte that
is using the serial poll or querying the event register with the summary bit that caused the service
request will clear the request service.
To use a serial poll:
You can send the serial poll message to read the Status Byte Summary Register. The serial poll will
clear the RQS bit automatically in the Status Byte Summary Register, but no other bits will be
affected.
Since synchronization between the bus controller program and the instrument is not guaranteed by
the IEEE-488.2 standard, you can send the *OPC? command to make sure that commands are
completed. Using a serial poll before *RST, *CLS, or other commands have completed can cause
reported conditions to be reported again.