3
GPIB Operation
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Serial Poll
Serial polling takes place once the SRQ interrupt line has been
asserted. The controller examines which instrument has generated
the interrupt by inspecting the SRQ bit in the STB register of each
instrument. Because service request is based on an interrupt mech-
anism, serial polling offers a reasonable compromise in terms of
servicing speed in multiple device configurations.
In the following example, the command "INE 1" enables the
event "new signal acquired" to be reported in the INR to the INB
bit of the status byte STB. The command "*SRE 1" enables the
INB of the status byte to generate an SRQ whenever it is set. The
function call IBWAIT instructs the computer to wait until one of
three conditions occur: &Hg000 in the mask (MASK%) corre-
sponds to a GPIB error, &H4000 to a time-out error and &H0800
to the detection of RQS (ReQuest for Service generated by the
SRQ bit).
Whenever IBWAIT detects RQS it automatically performs a serial
poll to find out which instrument generated the interrupt. It will
only exit if there was a time-out or if the instrument "SCOPE%"
generated SRQ. The additional function call IBRSP fetches the
value of the status byte which may be further interpreted. For this
example to function properly the value of ’Disable Auto Serial
Polling’ must be set ’off’ in the GPIB handler (use IBCONF.EXE
to check).
CMD$="*CLS; INE 1; *SRE 1"
CALL IBWRT(SCOPE%,CMD$)
MASK% = &HCg00
CALL IBWAIT(SCOPE%,MASK%)
IF (IBSTA% AND &HC000) <> 0 THEN PRINT "GPIB
Time-out Error" : STOP
CALL IBRSP(SCOPE%,SPR%)
PRINT "Status Byte = ", SPR%
Note: After the serial poll is completed, the RQS bit in the STB
status register is cleared. Note that the other STB register bits
remain set until they are cleared by means of a "* CLS" command
or the instrument is reset. If these bits are not cleared, they cannot
generate another interrupt.
Serial polling is only an advantage if there are several instruments
that may need attention. Board-level function calls can deal simul-
taneously with several instruments attached to the same interface
board. Refer to the National Instruments user’s manual.
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