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SECTION 3
IEEE-488 PROGRAMMING
3.1 INTRODUCTION
The IEEE-468 bus is an instrumentation
data bus with hard-
ware and progra mming standards originally
adopted by the
IEEE (Institute of Electrical and Electronic Engineers) in 1975
and given the IEEE488 designation. In 1978, standards were
upgraded into the IEEE488-1978 standards. The Model 193A
conforms
to these standards.
This section contains general bus information
as well as the
necessary programming
information
and is divided into the
following
sections:
1. Introductory
information
pertaining
to the IEEE-488 bus
in general is located in paragraphs
3.2 through
3.6.
2. Information
necessary to connect the Model 193A to the
IEEE-488 bus is contained
in paragraphs 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 paragraph 3.10. These are the most important
commands associated with the Model 193A as they con-
trol most of the instrument
functions.
5. Additional
information
necessary to use the Model 193A
over the IEEE-488 bus is located in the remaining
paragraphs.
3.2 BUS DESCRIPTION
The IEEE-488 bus, which is also frequently
referred to as
the GPIB (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 Figure
3-l. Generally,
a system will contain one controller
and a
number of other instruments
to which the commands arc
given. Device operation is categorized into three operators:
controller,
talker and listener. The controller
does what its
name implies; it controls the instruments
on the bus. The
talker sends data while a listener receives data. Depending
on the type of instrument,
any particular
device can br a
talker only, a listener only or both a talker and a listener.
There are two categories of controllers:
system controller,
and basic controller.
Both are able to control
other in-
struments, 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 cow
troller to another.
The IEEE-488 bus is limited to 15 deices, including
the con-
troller. Thus, any number of talkers and listeners up to that
limit may be present on the bus at one time. Although
several devices may be commanded to listen simultaneous-
ly, the bus can have only one active talker, or communica-
tions would be scrambled.
A device is placed in the talk or listen state by sending .m
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 30, 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 520. For exam-
ple, if the primary address is 10 ($OA), the actual listen .Id-
dress is $2A ($2A = $OA + $20). In a similar manner, the
talk address is obtained by OKing the primary address with
$40. With the present
example,
the talk address de-
rived from a primary address of 10 decimal would be S4A
($4A = $OA + $40).
The IEEE-488 standards also include another addressing
mode called secondary addressing.
Secondary addresses
lit in the range of $60.$7F, Note, hwvever that many dcviws
do not use secondary addressing.
Once a device is addressed to talk or listen, the appropriate
bus transactions take place. For example: if the Model 1Y3A
is addressed to talk, it places its data string on the bus one
byte at a time. The controller reads the information
and th<,
appropriate
software can be used to direct thtz iniormltiorr
to the desired location.
3-1
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