1250 Frequency Response Analyzer The RS423 and GPIB Ports. Remote Control
AMK/1250/4 13-25
13.13 SERIAL POLL/PARALLEL POLL
The 1250 can be configured to interrupt other GPIB activities by asserting a Service
Request (SRQ) when a particular event has occurred, e.g. End of Sweep or Data Ready.
When the Controller detects that SRQ has been asserted, it may "poll" the bus to find
out which device requires service. Polling can be either "Parallel", where all the devices
on the bus are interrogated simultaneously, or "Serial", where each device is
interrogated in turn.
Parallel Poll is faster than Serial for identification purposes, but provides less
information. However, choice of mode may be determined by the Controller since some
Controllers offer both forms of Poll, some offer only one, and some do not offer either.
Where both forms of Poll are available, and there are many devices connected to the
bus, it may be faster to perform a Parallel Poll to identify the device requesting service,
followed by a Serial Poll to ascertain the cause of the request.
13.13.1 SERIAL POLL
The GPIB Interface includes an 8-bit register which holds the STATUS BYTE (STB).
The STB is interrogated when the Controller initiates a Serial Poll, and contains the
following status data:
128
DATA
READY
64
RQS
32
END OF
PROGRAM
16
END
OF
FILE
8
END
OF
PLOT
4
END OF
SWEEP
2
END OF
MEASURE
1
ERROR
When the relevant condition exists, e.g. "End of Sweep", that bit will be set to "1".
Note that the status of all the bits, except "1" and "64", will continue to follow data
changes, e.g. if the 1250 is on RECYCLE, bit 2 will be set to "1" as each Measurement
is completed, then revert to "0" as a new Measurement commences.
The RQS (Request Service) bit 64 is set when the 1250 Interface generates an SRQ
(Service Request). A "1" in the RQS bit tells the Controller that this device generated
the SRQ. All 8 bits of the STB appear on the DI/O lines when the 1250 is interrogated.
The STB bits can be configured so that any one of several events can set the RQS bit,
as explained later in this section. However, to avoid possible ambiguities, it is
preferable to configure for one event only. To monitor several of the STB bits
simultaneously, the Controller can be instructed to Serial Poll continuously without using
SRQ.
The remaining seven bits identify which event or events caused the SRQ. They are:
Bit 128, Data Ready, which is set when data is available for ASCII or Binary output to
the GPIB.
Bits 32, 16, 8, 4 and 2, which indicate respectively the status of the Program, the File,
the Plotter (GPIB and Analog), the Sweep and the current Measurement. The "End of
File" bit 16 is set when either
a. The nth reading is stored in the File, where 'n' is the specified File size in Blocks (99
if default value).
or
b. The final reading, whatever number this may be, is read from the File. For example,
if the File contains only six entries, then bit 16 will be set to "1" when the 6th entry
has been read. Bit 1, the Error bit, which will be set when an error affecting the other
bits in the STB is detected. It does not register all possible errors in the 1250.
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