Section 4 Systems Application Via The Ieee Interface - Datron 1061A User Handbook Manual

SECTION
4
SYSTEMS APPLICATION
VIA
THE
IEEE
INTERFACE
The
IEEE
interface
option allows the 1061
and 1071
instruments to
form part
of a system, outputting
measurement data
to
other
parts of that system. In
addition, the
DVM
can be
instructed via the inter-
face
so
that
the instrument's
facilities
can be
selected
remotely.
In
order
that
instruments from differing
manufacturers can be
built
into the
same
system,
it
is
necessary
that all
interfaces
are compatible.
To
ensure
this,
the interfaces
conform
to
a standard
specification as detailed in
the publication
ANSI/
IEEE
Std 488-1978
called
'IEEE Standard
Digital
Interface for
Programmable
Instrumentation'.
A
typical
system
is
shown
in Fig. 4.1.
The
system
is
driven by
a controlling
device able to issue
commands
(controller),
receive
data
(listener)
and
output data
(talker).
The
DVM
is
able to receive
programming information
(listener)
and
to
output
data
(talker).
A
device
such
as
a
printer
or
VDU
will
simply input data
(listener)
its
output not being
into the system,
but onto paper or
the screen.
The
signal
scanner
is
also a listener only, receiving
only
commands.
Neither
its
signal
input or output are
directly
connected to the interface bus.
If
a
system comprises
several instruments, the
controller
is
able to
communicate
with the
instruments
individually
through
the
assignment to
each
of a different 'address'.
The
controller
adds
information
to the
address to define either talk
or
listen.
In the
system
example
(Fig.
4.1),
the
sequence
of
events for the
task of selection of
one
of the input
signals,
measuring
it
with the
DVM
and printing the
result
is
as follows:
1
.
The
controller requires to select
a signal
and
therefore
must send instructions
to
the
scanner.
The
instruction
must
not be received
by
the
DVM
or
the
printer
and
so
the
controller
sends the
general
bus message
"unlisten".
2.
To
enable the scanner to
receive
its
instructions
the controller
sends the
listen
address
which
has been assigned uniquely
to this
device.
It
follows this
with
the instructions required to
select
a channel.
The
instructions are
passed
along
the
IEEE
bus data
lines
as
coded
messages
(bytes).
The
code
most normally used
is
ASCII (American Standard
Code
for
Information Interchange).
3.
Since
the
scanner will
take a period of
time
to
change channels
it
sends a
message back to
the controller via
one
of
the
IEEE
bus
manage-
ment
lines
(SRQ) upon
completion.
Note
that
the scanner does not have
to
be addressed
as
a
talker
to
return
information
to
the controller
via the
management
lines.
This
is
only
necessary
if
the data
lines
are to
be used.
4.
The
controller
does not
know
which
of the
devices generated a
message on
this
manage-
ment
line,
since
all
devices are
connected
to the
same
line.
To
determine
the originator, the
controller
will,
by
sending messages
via the
interface,
ask or
'poll' all
the devices either
one
by one
(serial poll)
or together (parallel poll).
5.
The
controller
will
determine
that the
scanner
is
the
source
and must
send instructions to
the
DVM
so
that
the correct range
and
function
is
selected
before the
measurement
is
made.
Firstly
it
must
ensure
the
scanner
is
not
listening since
any coded messages
sent to the
DVM
(known
as "device
dependant") could be
misinterpreted
by
the scanner.
Sending
'unlisten'
followed by
the listen
address of the
DVM
and
the required
programming
instructions
achieves the desired
result.
6.
The
DVM
requires
a period of time
to
take
a
measurement
and prepare data.
It
generates a
message
via the
same management
line
(SRQ)
to the controller
when
it is
ready.
7.
The
controller
must
again
determine
which
of
the devices
sent
the
message
(service
request
SRQ)
by conducting a
poll.
8.
With
the
reading
available,
the controller
activates the printer
with
its
listen
address
only,
and
the
DVM
with
its
talk
address.
9.
When
the controller signals the
beginning of
the
transfer,
using
another
of the
bus
management
lines,
the
DVM
will
send the
4-1