Introduction; Bus Description - Keithley 775A Instruction Manual

3.1
INTRODUCTION
The
IEEE-488 bus
is
an instavmentation data bus with
stan-
dards adopted by
the
IEEE
(Institute of Electrical
and
Elec-
tronic
Engineers)
in
1975
and given the IEEE-488 designa-
tion.
The most
recent revision
of
bus standards
was made
in 1978;
hence
the
complete
description for
current bus
standards
is
the IEEE-488-1978 designation.
The Model
775
conforms
to
1978 standards.
This section contains general
bus information
as
well as
detailed
programming
information
and
is
divided
as
follows;
1.
General
intix)ductory
information pertaining
to
the
IEEE-488
bus
may
be found
primarily
in
paragraphs
3.2
ihrt^ugh
3.6.
2.
information necessary
to
connect the
Model
77S
to
the
bus
Is
contained
in
paragraph
3.7
and
3,8,
3.
General bus
command
programming
is
covered
in
paragraph
3.9,
4.
Device-dependent
command
programming
is
des-
cribed
in
detail in
paragraph
3.10,
The command.?
outlined
in this section
can be considered
to
be
the
most
important since they control
virtually
all
instrument
functions.
5.
Additional information-pertaining
to front
pane!
error
messages and
controller
programs can be
found
in
paragraphs
3.11
and
3.12.
3,2
BUS DESCRIPTION
The IEEE'488 bus
as
designed
as a parallel data transfer
medium
to
optimize data
transfer
without using an
ex-
cessive
number
of buslines.
In
keeping with
this goal,
the
bus has only eight data
lines that
are
used
for
both data
and most
commands.
Five
bus
management
lines
and
three
handshake
lines
round
out
the
complement
of
.signal
lines,
Since the
bus
is
of parallel
design,
all
devices con-
nected
to
the
bus have
the
same
information
avail, :^bie
simultaneously. Exactly
what
is
done
with
the information
by
each device
depends on
many
factors,
including device
capabilitie,3.
,A
typical
bus
configuration
for
controlled opcravjon
is
shown
in
Figure
.3-1.
The
typical
system
w-ill
have
o!u=
coi'!-
troiier
and.
one
or
more
instruments
to
which
commands'
are
given and,
in
most
cases,
from which
data
is
roceix'od.
Generally, there are three categories that describe dcvu:v
operation, 'fhese designations include;
controller; talker;
listener.
The
controller
does
what
its
name
implies;
it
control,?
other
devices
on
the bus,
A
talker
sends
data,
while
a listener
receives data,
Depending on
the instrument,
a parttcular
device
may
be
a
talker only,
a
listener only, or
both
a tdlker
and
listener.
.Any
given sy.stem
can have only one
controller
(ccintroi
may
be passed
to
an appropriate device
through
a
.specuh
command),
but
any
number
of talkers or listeners nrav be
present
up
to
the
hardware
constraints of the bus. General-
ly,
the
bus
is
limited
to 15 devices,
but
this
number
snav
he
reduced
if
higher than normal
data transfer
rates
.;'.re
required or
if
longer
than normal
cables
are
used,
Several
devices
may
be
commanded
to iisten at uru:v, but
only
one
device
may
be
a talker at
any
given
time.
Other-
wise,
communicationis
would
be scrambled.