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Operation
Model 8902A
Status Byte
Overview
The Status Byte bits 0-5 reflect the status of the unmasked Instrument Conditions. Refer to Table
3-9, "SRQ Mask and Status Byte Bit Descriptions" for bit definitions. At power-up and Instrument
Preset, the only Status Byte bit which is unmasked is the HP-IB Error Code bit.
The RQS bit (bit 6) is automatically set if any other bit in the Status Byte is set true. When the RQS
bit is set, the Service Request message is sent. The front-panel SRQ annunciator turns on at the next
front-panel display update. Once any Status Byte bit has been set, it remains set until the Status Byte
is read.
If one of these Instrument Conditions occurs, while it is masked by the SRQ Mask, NEITHER the
Status Byte bit corresponding to the condition NOR the RQS bit will be set, and the Service Request
message will not be sent.
Reading the Status Byte
NOTE
0
A
Statirs Byte Bit that
is not
irnmnskcd by the SRQ Mask will AL WA YS
read as
u
zt=ro when the Statirs Byte
is
read. Only an irnmasked bit can
be read
as
a
one,
and then only ( f i t s initiating Instrirment Condition
occiirs AFTER
it is irnmasked.
0
Reading the Statirs Byte is the only accirrate way to determine the
condition of the SRQ biis line (Service Request message). The SRQ
annrinciafor is only irpdafcd as the front panel is updated. If is NOT
ticd directly
to
the SRQ
hiis
h e .
0
The Measuring Rtwivtv cannot
a k r
the Sfatirs Byte while in stjrial poll
mode; thertgore, if is not possibk to confinrrally poll the Statirs Byfe while
waitingjor
a
condifion
fo
caiise
a bit to
be
s d .
a
Rcading Stuf
11s
o f non-cxisftw
f
dcvim (HP-IB addrc.ssc>s not
in
i r s e )
will
hang the HP 8902A HP-IB and rtyirircs
a
Device Clear (DCL) message
to clear the birs.
The Status Byte can be read using a serial poll, only if the instrument sends a Service Request
Message. Only those bits which are unmasked by the SRQ Mask can be read accurately.
Talk Status Only causes the instrument to only output the Status Byte. Refer to "TALK and LISTEN
Status Descriptions" for more information.
3-32
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