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4.2.2
The
signal
lines
shown
in
Figure
4.2
consist
of three functionally separate
sets:
Data,
Handshake, and
Interface.
4.
2.
2.1
Data
-
the
data
lines
consist
of
DIO-1
to
DIO-8.
These
lines
are
the signal
channels
over
which data
flows
between
all
instruments on
the
bus
in
bit-parallel,
byte-
serial
form.
4. 2. 2.
2
Handshake
-
these three transfer
lines
consist
of:
DAV
(Data
Valid),
NDAC
(Not
Data
Accepted),
and
NRFD
(Not
Ready
for
Data).
These
lines
provide
communication between GPIB
bus
members
(i.e.,
between
the
instrument
that
is
talking
and
the instrument(s) that are
listening) to
synchronize
the
information flow
across the
eight
data
lines.
These
lines
derive
their
nomenclature from
their
meaning
in
the
low
or
1
state
(e.g.,
when
NRFD
is
low, the device
is
Not Ready
for
Data).
a.
DAV
-
when
low,
it
signifies that valid
information
is
available
on
the
data
lines
b.
NRFD
-
when
low,
it
signifies that
the
instrument
is
not ready
to
accept information
c.
NDAC-
when
low,
it
signifies that
information
is
not
accepted by
the
acceptor bus device
4.2.
.
3
Interface
-
these
five
interface
lines
coordinate the information flow on the
bus.
a.
IFC
(Interface
Clear)
-
places
the
instrument
in
the
Idle
state
(i.e.,
Untalk, Unlisten)
b.
ATN
(Attention)
-
indicates the kind of
information on
the
data
lines
during a
handshake
transfer
sequence.
Low
indicates
data
lines
carry
interface
commands;
high indicates that the
data
lines
carry data
c.
REN
(Remote
Enable)
-
arms
the
instrument
to
select
Remote
operation
when
it's
addressed
as a listener
d.
SRQ
(Service
Request)
- signals the
system
controller that a peripheral
device
or
bus
member
wants
attention
for
purposes such
as
transmitting
measurement,
status, or
condition information
to
the
system
controller
e.
EOI
(End
or Identify)
-
used
for
(1)
signifying the
end
of
a
message and
(2)
together with
ATN,
signalling
bus peripherals
to
set the I/O
bit
assigned
for parallel poll identification
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