HP IBASIC Supplement 8-17
Interfacing with External Devices
The IBASIC GPIB Model
The IBASIC GPIB Model
The fact that IBASIC resides in, and coexists with an instrument poses a
large set of possible interactions, both internal to the instrument and
externally with other controllers and instruments. This section defines
the principal players and rules of order when IBASIC is running within
the host instrument.
The External Bus and the Internal Bus
There is physically only one GPIB port and one GPIB address for the
analyzer. IBASIC has access to two GPIB ports: the “real” external port
(select code 7) and a “virtual” internal port (select code 8), through which
it communicates with the analyzer.
The analyzer has only one output buffer, one input buffer, and one set of
status registers. Commands and data from both ports are placed in the
same input buffer and data read out of both ports comes from the same
output buffer. The instrument will not provide any kind of arbitration
between an external controller and an IBASIC program.
The analyzer always behaves as if there is only one controller. If an
IBASIC program is running, it is assumed to be the controller and
therefore will receive all SRQs from the host instrument (via the internal
port).
Service Request Indicators
An external controller may perform a serial poll (SPOLL) at any time
without affecting a running IBASIC program. There are two Service
Request Indicators (SRI) — one for the external port and one for the
internal port. The internal SRI can only be cleared by an IBASIC
program performing an SPOLL on device 800. The external SRI can only
be cleared by an SPOLL from an external controller and can only be set
when there is not an active IBASIC program.
The two SRIs will be set to their OR'd value when a program starts, and
again when it finishes. This assures that any pending SRQs can be
serviced by the instrument's new controller.
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