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Service
Request
the
4708
can asynchronously
request service
from
the controller
by
putting the
SRQ
line true
(low).
SRQ
is
always
generated
by
the action
of switching
the
4708
power
ON,
as the
power-up
default
mode
is
Q0.
A
user
can
program
the
4708
to
generate
SRQs
(or not)
using
command
code
Q:
Code
Q0
-
SRQ
on any
of
the
states in
Table
5.5
Q1
-
SRQ
on
overload
and any
FAIL
state
in
Table
5.5 (but
not
in
Error
states).
Q2
.
No SRQs
generated
Serial Poll
and
RQS
Status Byte
If
programmed
for
SRQ
response, the
bus
controller will
pause
in
its
operation
to
attend
to
the service request.
It
first
conducts
a
serial
or
parallel poll to
determine
which
device
initiated
the
SRQ. The
4708
does
not
react
to parallel poll,
but only
to
serial
poll,
during
which
each device
is
addressed
in turn.
The
instrument responds
to
its
serial
poll
address
by
releasing
a
prepared
'RQS
Status
Byte' onto the
bus.
The
RQS
Request
Bit
(bit
B7
of
its
status
byte)
is
asserted
only
if
the
4708
has
generated
the
SRQ.
This
validates the
remainder of
the byte,
which
describes the causal condition
by
the
state
codes
listed in
Table
5.5.
RQS
Status
bit
b8:
bit
b7:
bit
b6
true:
bit
b6
false:
Byte
Composition
Indicates a
syntax or option
error
when
true.
The
RQS
request
bit,
when
true,
confirms
that
the
4708 was
the
SRQ
originator.
The
RQS
status
byte
is
not
valid unless
bit
b7
is
true.
Each
combination of
bits
b5-bl
represents
a
single
state
as
listed
in
Table
5.5.
Bits
b5-bl each
represent separate functional
states
within
the
4708 and
rthe
RQS
byte
repre-
sents several states
as
listed in
Table
5.5.
Example
with
bit
b6
false:
RQS
status
byte
OKWOOOl
represents:
0
-
No
option or
Syntax
error
1
-
This instrument
originated the
SRQ
0
-
The
following
bits
each
represent separate
states
0
-
This
bit
is
not
used
in
the
4708
0
-
No
High
Voltage warning
0
-
Auxiliary
register
not
at
limit
0
-
Main
register
not
at
limit
1
-
Output
is
ON
The
RQS
status
byte should not
be
confused with
other
status
messages
(e.g.
'calibrator'
or 'software'
status,
described
earlier
under 'RecalWerify') which
arc called
up by
the
system con-
troller's
program.
JS
status
bytes
which
may
be
transmitted
by
the
jn
In
the
byte
Is
valid
only
If
bit
7
(request
bit) Is true.
or
lest-for-service bit
4
ister
limit
reached
Register
limit
reached
age
Warning
ssage
available
)ecification
not displayable
VL
mode: Output
not
ON
AL
mode:
Incorrect
Range/Function
AL
mode:
Insufficient store
span
Tor or Offset
mode:
Overscale
output requested
AL
mode:
Resistance
selected
exceeds
val.
value
C
Functions:
Output
has
been
limited
by
internal
constraints
eneral selection error
ption
not
fitted
lit
condition
rectified
3r-temperature
jr-voltage
itrol
data corrupted
cision divider
fault
ety
alarm
.
store
sum
check
non-parity
)V
power
supply
fault
(automatically
resets
if
temporary)
power
supply
fault
/
In-guard
power
supply
fault
odel
4600 communication
fault
istrument
reset to
power-up
state
External
frequency
no
present
uency
not
calibrated
-
Current or Voltage
limit
r(iV)S0T0W0
>ower-up
but
K
=
and
L
=
continue
unchanged
tivates
preceding
string.
DIO
Line
Transmissions
Providing
Q0
or
Q1
has
been
selected;
when
the
4708
has a
message
to
transmit
over
the
DIO
lines,
it
sets
the
SRQ
line true
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