Keithley 237

Introduction
Basically,
the
IEEE-488
bus
is simply a communication
system between
two
or
more electronic devices. A device
can
be
either
an
instrument
or
a computer.
When
a
com-
puter
is
used
on
the
bus,
it
serves to supervise
the
com-
munication exchange between all
the
devices
and
is
known
as
the
controller. Supervision
by
the
controller
consists
of
determining which device
will
talk
and
which
device
will
listen. As a talker, a device will
output
infor-
mation
and
as a listener, a device
will
receive
informa-
tion. To simplify
the
task
of
keeping track
of
the
devices, a
unique
address
number
is assigned to each one.
On
the bus, only one device can talk
at
a time
and
is
ad-
dressed to talk
by
the
controller. The device
that
is talking
is
known
as
the
active talker. The devices
that
need
to lis-
ten to the talker are addressed to listen
by
the
controller.
Each listener is
then
referred to as
an
active listener. De-
vices
that
do
not
need
to listen
are
instructed to unlisten.
The reason for
the
unlisten instruction is to optimize
the
speed
of
bus
information transfer since
the
task
of
listen-
ing
takes
up
bus
time.
Through the
use
of
control lines, a
handshake
sequence
takes place
in
the
transfer process
of
information from a
talker to a listener. This handshake sequence helps
en-
sure
the
credibility
of
the
information transfer.
The
basic
handshake sequence between
an
active controller
(talker)
and
a listener is as follows:
1.
The listener indicates
that
it
is
read
to listen.
2.
The talker places
the
byte
of
data
on
the
bus
and
indicates
that
the
data
is available to
the
listener.
3.
The listener,
aware
that
the
data
is available,
ac-
cepts
the
data
and
then
indicates
that
the~ta
has
been accepted.
4.
The talker,
aware
that
the
data
has
been accepted,
stops sending
data
and
indicates
that
data
is
not
being, sent.
5.
The listener, aware
that
there is
no
data
on
the
bus,
indicates
that
it
is
ready
for the next byte
of
data.
+
+
APPENDIX
E
IEEE-488 Bus
Overview
Bus
Description
The IEEE-488 bus, which is also frequently referred to a
the
GPffi (General Purpose Interface Bus),
was
designed
as a parallel transfer
medium
to optimize
data
transfer
without
using
an
excessive
number
of
bus
lines.
In
keep-
ing
with
this
goal,
the
bus
has only eight
data
lines
that
are
used
for
both
data
and
with
most
commands. Five
bus
management
lines
and
three
handshake
lines
round
out
the
complement
of
bus
signal lines.
A typical
set
up
for controlled operation is
shown
in
Fig-
ure
E-1. Generally, a system
will
contain one controller
and
a
number
of
other
instruments to which the
com-
mands
are given. Device operation is categorized into
three operators: controller, talker
and
listener. The con-
troller does
what
its
name
implies;
it
controls
the
instru-
ments
on
the
bus. The talker
sends
data
while a listener
receives data.
Depending
on
the
type
of
instrument,
any
particular device can
be
a talker only, a listener only
or
both
a talker
and
listener.
There are
two
categories
of
controllers: system controller,
and
basic controller. Both are able to control other
instru-
ments,
but
only the system controller
has
the absolute
authority
in
the
system.
In
a system
with
more
than
one
controller, only
one
controller
may
be
active
at
any given
time. Certain protocol is
used
to pass control from one
controller to another.
The IEEE-488
bus
is limited to
15
devices, including the
controller. Thus,
any
number
of
talkers
and
listeners
up
to
that
limit
may
be
present
on
the
bus
at
one
time.
Al-
though
several devices
may
be
commanded
to listen
si-
multaneously,
the
bus
can
have
only
one
active talker,
or
communications
would
be
scrambled.
A device is placed
in
the
talk
or
listen state
by
sending
an
appropriate talk
or
listen command. These talk
and
listen
commands
are
derived
from
an
instrument's
primary
ad-
dress. The
primary
address
may
have
any
value between
0
and
31,
and
is generally set
by
rear
panel
DIP switches
or
programmed
in
from
the
front
panel
of
the
instrument.
The actual listen
address
value sent
out
over the
bus
is
ob-
tained
by
ORing the
primary
address
with
$20. For exam-
E-1