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Figure
3*1.
^EEE
Bus
Configuration
Before
device
Cfin
talker
listen,
it
must
be appropriately
addressed. Devices
are
selected, on.
the basis
of their
primary
address; the
addressed
device
is
sent
a talk
or
itsten
command
derived
from
its
primary
address.
Nor-
mally,
each
device
on
the
bus
has
a
unique primary
ad-
dresvS
.so
that
each
may
be
addressed
individually.
The
bus
also
has another addressing
mode
called
secondary
ad-
dressing, but not
all
devices
use
this
addresvsing
mode.
Once
the device
is
addressed
to talk
or
listen,
appropriate
bus
transaction.^
are set to
take
place,
R>r example,
if
an
instrument
is
addressed
to
talk,
it
will
usually place
its
data
on
the
bus
one
byte
at
a time.
The
listening
device
will
then read
this
information,
and
the appropriate software
can then be
used
to
channel
the informatton
to
the
desired,
location.
3.3
IEEE4B8
BUS
LINES
The
signal lines
on
the
IEEE-488 bus
are
grouped
into three
gener^
categories.
The
data
lines
handle bus
information,
while
the
handshake and bus management
lines
ensure
that
proper
data
transfer
and bus
operation
takes
place;
Each
of
the
bus
lines
is
active
low
so
that
approximately
7,ero
volts
is
a
logic
one.
The
follov/ing
paragraph.? describe
the
purpose
of
these
lines,
which
are
shown
in
Figure
3-1.
inagement
Lines
Tlie
bus
management
group
is
made,
up
of
five
signal
lines
that
help ensure
an
orderly
transfer of data.
These
lines
the uniline
commands
described
in
ATN
(Attention)—
The
ATN
line
is
one
of the
more
impor-
tant
management
lines.
The
state
of
the
ATN
li.ne
deter-
mines whether
controller
information
on
the
data bus
is
to
be considered
data or a
multilirie
command
as des-
cribed
in
paragraph
3.4.
!FC
{Interlace
Qear)~Setting
the
IFC
line
true
(low)
cau.ses
the
bus
to
go
to a
known
state.
REN
(Remote Enable)—
Setting the
REN
lirie
low sends
the
REN
command.
This
sets
up
instruments
on
the
bus
for
remote
operation.
EOI
(End
Or
Identify)—
The EOI
line
is
used
to
send
the
EOI
command
that
usually terminates
a
multi-byte
transfer
.sequence,.
SRQ
(Seiwice
Request)—
The
SRQ
line
is
set
low
by a device
when
It
requires
.service
from
the
controller.
3.3.2
Handshaka
Lines
The
bus
uses
three
handshake
lines that
operate
in
an
in-
terlocked
sequence. This
method
ensures
reliable
data
transfer
regardless
of
the
transfer
rate.
Generally, data
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