GPIB
370B User Manual 4-43
Service Requests
The standard GPIB status and error reporting system used by the 370B sends
interrupt messages to the bus controller by asserting the Service Request (SRQ)
line on the bus. This SRQ message indicates that either an error or a change in
status has occurred.
To service an interrupt, the controller “polls” the instruments on the bus. The
instrument asserting SRQ, the 370B in this case, returns a status byte indicating
the category of the event causing the Service Request. Each Service Request is
automatically cleared after responding to the poll. If there is more than one event
to report, the instrument re–asserts SRQ until all pending events have been
reported. A complete list of status bytes that can occur is found in Table 4–12.
After polling the 370B for the status byte, the controller can obtain more detailed
information about the Service Request event by sending the EVEnt? query. The
response to an EVEnt? query is an event code, which is a <NR1> number
corresponding to certain conditions that may have occurred. Table 4–13 lists the
event codes returned by the 370B.
The status byte and event code can be accessed later if they are not read and
cleared immediately. In the case of multiple events, only the latest status byte
and the one pending are saved. Event codes, however, are kept in a ten-deep
Last-In-First-Out (LIFO) buffer for later recall.
The following program segments demonstrate how Service Requests are handled
along with the corresponding status byte and event code. The status byte and
event code are printed on the controller display to track instrument status.
IBM PC: (This is for use with the auto-serial-poll flag disabled in the National
Instruments GPIB card configuration file.)
800 REM *** SIMPLE SRQ HANDLER FOR 370B ***
810 CALL IBRSP(BD%,SPR%)
820 WRT$=”EVENT?”
830 CALL IBWRT(BD%,WRT$)
840 RD$=SPACE$(100)
850 CALL IBRD(BD%,RD$)
860 PRINT ”STATUS=”;SPR%,”EVENT=”;RD$
Hewlett Packard 200/300 Series:
800 REM *** SIMPLE SRQ HANDLER FOR 370B ***
810 STB=SPOLL(DEV)! Poll device previously defined.
820 OUTPUT DEV;”EVENT?”,END ! Send ”EVENT?” query.
830 ENTER DEV;EVENT$ ! Input response.
840 PRINT ”STATUS= ”;STB;” EVENT=”;EVENT$ ! Show status and
event.
Handling Service
Requests
To Next Page
Loading...
General