IEEE-488 Reference
3-18
The individual bits of the Service Request Enable Register
can be set or cleared by using the following common com-
mand (see paragraph 3.10.11 for details):
*SRE <NRf>
The Service Request Enable Register can be read using the
following common query command (see paragraph 3.10.11
for details):
*SRE?
The Service Request Enable Register is not cleared when it
is read using the *SRE? query command. The Service
Request Enable Register clears when power is cycled or a
parameter (n) value of zero is sent with the *SRE command
(*SRE 0).
Serial poll and SRQ
Any enabled event summary bit that goes from 0 to 1 will set
RQS and generate a service request (SRQ). The user’s test
program can periodically read the Status Byte Register to
check to see if a service request (SRQ) has occurred and
what caused it. In the event of an SRQ, the program can, for
example, branch to an appropriate subroutine that will ser-
vice the request. Typically, service requests (SRQs) are man-
aged by the serial poll sequence of the Model 2002. If an
SRQ does not occur, bit B6 (RQS) of the Status Byte Regis-
ter will remain cleared and the program will simply proceed
normally after the serial poll is performed. If an SRQ does
occur, bit B6 of the Status Byte Register will set and the pro-
gram can branch to a service subroutine when the SRQ is de-
tected by the serial poll.
The serial poll automatically resets RQS of the Status Byte
Register. This will allow subsequent serial polls to monitor
Bit 6 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 which caused the first SRQ has not
been cleared.
A serial poll clears RQS but does not clear MSS. The MSS
bit will stay set until all Status Byte event summary bits are
cleared.
The following QuickBASIC 4.5 program (using the KPC-
488.2 interface and the CECHP driver) demonstrates how
serial poll can be used to detect an SRQ.
CLS
OPEN "ieee" FOR OUTPUT AS #1
OPEN "ieee" FOR INPUT AS #2
PRINT #1, "output 16; *cls" ' Clear Status Byte
Register
PRINT #1, "output 16; *ese 32" ' Unmask command
errors
PRINT #1, "output 16; *sre 32" ' Unmask event sum-
mary message
PRINT #1, "output 16; *ese" ' Error - missing
parameter
SLEEP 1
PRINT #1, "SPOLL 16" ' Serial poll 2002
INPUT #2, S ' Read Status Byte
Register
S = S OR 191 ' OR register with a
mask
IF S = 255 THEN
GOSUB srq ' Go to subroutine
to acknowledge
SRQ
END IF
PRINT
END
srq:
PRINT "SRQ Has Occurred – RQS (bit B6) is set (1)"
RETURN
3.8 Trigger model (IEEE-488 operation)
The following information describes the operation process of
the Model 2002 over the IEEE-488 bus. The flowchart in
Figure 3-13, which summarizes operation over the bus, is
called the Trigger model. It is called the trigger model
because operation is controlled by SCPI commands from the
Trigger subsystem (see paragraph 3.23. Notice that key SCPI
commands are included in the trigger model.
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