Agilent Technologies HP 82335, Install Manual, Page: 46 / 54

HP-IS Fundamentals
Service Requests
Service Requests
Some devices that operate on the interface have the ability
(0
request service
from the system controller. A device may request service when it has
completed a measurement, when it has detected a critical condition, or under
many other circumstances.
A service request (SRQ)
is
initiated when the device sets the SRQ line true.
The controller, sensing that SRQ has IJI;:C!I
sct (tYlJil;:ully cither by polling
the
status of the line, or by enabling an SRQ interrupt), can poll devices in one
of two ways: seriaJ poll or parallel poll.
Serial Poll
A typical sequence of events in perfoming a serial poll is:
• Establish a device as a talker.
• Send SPE to set up Serial Poll mode.
•
Wait
for the addressed device to send its serial p o l
l
response byte.
• Send an SPD and UNT to disable the Serial Poll mode.
The meaning of the . serial poll response byte depends upon the individual
device. However, ifbit 6 of the response byte (bit value 64) is I , the device is
indicating it has requested service. lf bit 6 is
0,
the polled device is not the
one that requested service. lndividual device manuals provide additional
information on the meanings of serial poll response bytes.
Parallel Poll
Parallel JX>lling pennits the status of mUltiple devices on the HP-m to be checked
simultaneously. Each device is assigned a data line (DIO 1 through DI08) that the
device selS true during the parallel PJII routine if it requires service.
More than one device can be assigned to a particular data line. If a shared
line is sensed true. a serial po
ll
can typically be performed to determine
which device set the line. A parallel poll is started when the controller
asserts ATN and EOI together. After a short period of time. the controller
reads the poll byte and begins its interpretation thereof.
Some devices can be configured (by the PPC command) to respond on
specific data lines. Other devices may respond on Ijnes selected by switches
or jumpers in the devices. Some devices do not have parallel poll capability.
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