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SECTION 3
IEEE BUS OPERATION
3.1 INTRODUCTION
This section
contains
information
necessary
to operate
the
Model 705 over the IEEE-488 bus. The Model 705 has a stan-
dard IEEE-488 interface
that allows the user to give commands
and read data via an external
device. All front panel programs
and functions
are available
to the bus (except Programs 3, 90,
91 and 92).
A typical bus set up for controlled
operation
is shown in Figure
3-1.
Generally,
a system
will
contain
one controller
and a
number
of other
instruments
to which
the commands
are
given.
Device
operation
is categorized
into three operators:
controller,
talker
and listener.
The controller
does what
its
name implies; it controls
the instrument
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 a listener.
Any given system can have only one active controller,
but any
number of talkers or listeners may be presented
up to the hard-
ware limitations
of the bus. Generally,
the bus is limited to 15
devices,
but this number may be reduced if higher than normal
data transfer
rates are required or if longer than normal cables
are used.
Only one device on the bus may be a talker at any given time
while
several devices
can be commanded
to listen.
Before a
device
can talk or listen it must be appropriately
addressed.
Devices
are selected
by their primary
address.
Usually,
each
device
on the bus has its own primary
address so that each
device may be addressed
individually.
The primary
address of
the Model 705 is factory
set at 17. The factory
set primary ad-
dress may be changed
via front
panel
program
number
3.
R,efer to paragraph
2.6 for more information
about
the front
panel programs.
Once a device is addressed to talk or listen, the appropriate
bus
transactions
take place. For example,
if the Model 705 is ad-
dressed to talk, it places the data string on the bus one byte at
a time.
The
controller
reads the
information
and the ap-
propriate
software
can then be used to direct the information
to the desired location.
3.1.1 Software
Considerations
The
most
sophisticated
computer
in the world
would
be
useless without
the necessary
software.
This basic require-
ment is also true of the IEEE-488 bus, which requires the use of
handler
routines
as described
in this section.
Before a controller
can be used with the IEEE-488 interface,
the user must make certain that appropriate
handler software
is
present
within
the controller.
With the HP-85 computer,
for
example,
the HP-85 interface
card must be used with an addi-
tional
I/O ROM, which
contains
the necessary
handler
soft-
ware.
Other small computers
that can be used as controllers
have
limited
IEEE command
capability.
The PET/CBM
computers,
for example,
are incapable
of sending
multiline
commands
from BASIC,
although
these commands
can be sent through
machine-language
routines.
The capabilities
of other
small
computers
depends
on the particular
interface
being
used.
Often,
little
software
"tricks"
are required
to achieve
the
desired results.
From the preceding
discussion,
the message
is clear: make
sure the proper
software
is being
used with
the interface,
Often, the user may incorrectly
suspect
that a hardware
pro-
blem is causing fault, when it was the software
that was caus-
ing the problem
all along.
3.1.2 HP-85
BASIC
Statements
Many of the programming
instructions
covered
in Section
3
use examples
written
in Hewlett-Packard
Model
85 BASIC.
The HP-85 was chosen for these examples
because
it has a
large
number
of BASIC
statements
that
control
IEEE-488
operation.
This section covers those HP-85 BASIC statements
that are essential to Model 705 operation.
A complete
list of HP-85 BASIC statements
is shown
in Table
3-1. All the statements
in the table have one or three digit
arguments
that must be specified.
The first digit is the HP-85
interface
select code, which
is set to 7 at the factory.
The last
two digits of those statements
that require a three digit argu-
ment specify the primary address.
Generally,
only those com-
mands that actually require an address to be sent over the bus
require
that the primary
address
be specified
in the BASIC
statement.
Those
statements
in the table
with
three
digit
arguments
assume that the primary
address
of the device
is set at 17.
Other primary addresses require that the last two digits be set
to the corresponding
value. For example,
to send a GTL com-
mand to device 22, the following
BASIC statement
would
be
used: LOCAL 722.
Some of the statements
in the table have two forms; the exact
configuration
used depends on the desired command.
For ex-
ample, CLEAR 7 will cause a DCL to be sent, while CLEAR 717
causes an SDC to be transmitted
to device 717.
The third column of Table 3-l lists the mnemonics
for the com-
mand sequences.
While most of these are covered
elsewhere,
a couple of points should be noted. The ATN line is set low by
the controller
if the data bus contains
a multiline
command.
3-1
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