Datron 4708 User Handbook Manual page 51

Typical
System
A
typical
system
is
shown
in Fig.
5.2.
The
system
is
directed
by
a
controlling device able
to:
(a)
'Control'
(Issue
commands)
(b)
'Listen' (Receive
data)
and
(c)
'Talk' (Transmit
data)
EXAMPLE OF A SYSTEM
IN
OPERATION
In the
system example
(Fig. 5.2)
the
programme
task
could be
to
check
the
DMM
calibration against the
4708, and
print out the
results.
The
following
is
a
typical
squence of
events:
(1)
The
controller needs
to instruct the
4708
to set
its
output
to
a
calibration point for the
DMM.
These
commands
must
not
be
received
by
the
DMM
or the printer
and
so
the controller sends
the general bus message
'Unlisten'.
When
sending
general messages,
the controller
makes
all
bus
devices
interpret
any
DIO-line
data as configura-
tion or data-flow
commands, by
holding
the
ATN
line
true.
(2)
The
controller then sends the4708
listen
address
to
force
it
to receive, followed
by 4708
configuration
commands
(including the
Output
Disable message,
to
prevent
the
DMM
receiving
an
inappropriate analog
input).
The
in-
structions are passed along
the
DIO
(data input-output)
lines
as
coded messages
(bytes).
The
code
is
used
in
ASCII
(American
Std.
Code
for
Information
Inter-
change).
(3)
Although
the
4708
accepts the
instructions as they are
passed,
their
implementation
takes
a
short time.
The
controller
would
perform
other tasks during
this
period.
In the example,
it
would
pass configuring
commands
to
the
DMM,
after 'Unlisten'
and
the
DMM
listen address
have been
sent.
(4)
The
DMM
also needs time
to
settle
into stable operation,
so
the controller performs
other tasks while
waiting,
such
as configuring
the printer.
(5)
The
controller next generates
'Unlisten',
addresses the
4708
as
listener,
and
reconfigures
its
Analog Output
On
by an Output Enable
message.
If the
4708
has executed
its
previous
instructions,
it sets
OUTPUT
ON
immedi-
ately,
otherwise
the
OUTPUT
is
set
ON
as
soon
as
they
have been
executed. In either case, the
instrument sends
a
message back
to
the controller via the
SRQ
(Service
Request)
management
line, if
programmed
to
do
so.
(6)
As
the
SRQ
facility
is
available
to all
bus
devices (Wired-
OR
function), the controller needs
to
discover
which one
sent the
'SRQ'.
It
therefore asks
all
devices one by one
('serial poll'), finds out
that the
4708
is
the
SRQ
source
and
that
its
OUTPUT
is
ON.
(7) It
next addresses
the
DMM
as a
listener,
and
sends the
GET
message (Group Execute
Trigger) via the
DIO
lines to
initiate
the reading. After a
short delay
for
measurement,
the
DMM
prepares output data and
SRQ
's
the controller
when
it is
ready
for
transfer.
(8)
The
controller
identifies the
DMM
by
a
serial poll.
Finding
that
the reading
is
available,
it
sends
the
DMM's
talk address, and
printer's
listen address,
to
activate both
devices.
(9)
The
controller
sets the
ATN
line
false, thus releasing
both devices
to start the
transfer.
The
DMM
sends
its
data,
byte
by
byte, via the
DIO
lines to the
printer.
This
data
must be
in
a
form
acceptable
to the printer, and
to
ensure orderly
transfer,
each
byte
is
transferred by
'Handshake',
using the three
Transfer-Control
lines.
(10)
Usually
the controller
is
also listening to
this data
trans-
fer to
determine
when
it
is
complete.
As
an
aid
to the
controller
and
printer, the
DMM
can send
another mes-
sage with
the
last
byte
to
be
transferred
(EOI-end
or
identify,
using another bus management
line).
(1 1)
The
sequence
is
complete,
and
the controller
can
start
again
at
another
calibration point.
The
controller
holds
the
REN
line true
when
taking remote
control.
It
can send an addressed
command GTL,
or
some
controllers
can
set
REN
false, to
permit temporary
manual
control of a
device.
The IFC
line
is
used
at
the discretion
of the
controller, to clear
any
activity
off the bus.
Sequences such
as
this
are often assembled
into programs
to
check
DMMs
at
many
calibration points;
changing
functions,
ranges and
output
levels as
designed by
the
user.
The
program
would
also include
'display'
messages
to
complete
the
printout
in a recognizable form
for the user's convenience.
Programs
must
also cater for
FAIL
and
ERROR
SRQs.
Note
that
many
of
the individual steps detailed above
will
be
transparent
to the
programmer.
The
level
of transparency
will
be dependent on
the
controller.
Refer
to the relevant
documen-
tation for further information.
With
a
Datron Autocal
DMM,
other sequences can cause
the
DMM
errors to
be reduced
until
they
are
within
specification,
using
its
'calibrate'
mode.
Using
the
4708
in
a
System
ADDRESSING THE
4708
Bus
Address
The
instrument address
is
set
manually
using a six-way
mini-
ature
switch near
the interface connector on
the rear panel. Five
of
the switches
are
used
to
set
any
address
in the
range 00
to 30,
using a binary code.
5-3