Keithley 237

APPENDIX£
IEEE-488
Bus
Overview
TO
OTHER
DEVICES
DEVICE1
ABLE
TO
TALK.
LISTEN
AND
CONTROL
(COMPUTER)
DEVICE2
ABLE
TO
TALK
AND
LISTEN
(236/237)
DEVICE3
ONLY
ABLE
TO
LISTEN
(PRINTER)
DEVICE4
ONLY
ABLE
TO
TALK
A
ttttt
ttt
fr
~
~
A
~
1---
<
....
~
=
(
==
-
<
DATA
BUS
DATA
BYTE
TRANSFER
CONTROL
GENERAL
INTERFACE
MANAGEMENT
'---}
0101
...
80
ATA
S)
-
(8LINE
"-DAV
}
----NRFD
HANDSHA
-NDAC
KE
IFC
ATN
SRQ
BUS
MANA
GEM
REN
EOI
}
ENT
Figure
E-1.
IEEE-488
Bus
Configuration
ple,
if
the primary address
is
16 decimal
($10),
the actual
listen address
is
$30
($30
=
$10
+
$20).
In a similar manner,
the talk address
is
obtained
by
ORing the primary ad-
dress
with
$40.
With the present example, the talk ad-
dress derived from a primary address
of
16 decimal
would
be
$50 ($50= $10
+
$40).
The IEEE-488 standards also include another addressing
mode
called secondary addressing. Secondary addresses
lie
in
the range of $60-$7F.
Note, however,
that
many
de-
vices, including the Model236/237 /238,
do
not
use sec-
ondary addressing.
E-2
+
+
Once a device
is
addressed to talk
or
listen, the appropri-
ate
bus
transactions take place. For example:
if
the instru-
ment
is
addressed to talk,
it
places its
data
string
on
the
bus
one byte
at
a time. The controller reads the informa-
tion
and
the appropriate software can
be
used
to direct
the information to the desired location.
Bus
Lines
The signal lines
on
the IEEE-488
bus
are grouped into
three different categories:
data
lines, management lines
and
handshake lines. The
data
lines handle
bus
data
and
commands, while the management
and
handshake lines
ensure
that
proper
data
transfer
and
operation takes
place. Each
bus
line is active low,
with
approximately
zero volts representing a logic 1 (true). The following
paragraphs describe the operation of these lines .
Data
Lines
The IEEE-488
bus
uses eight
data
lines
that
transfer
data
one byte
at
a time.
DI01
(Data
Input/Output)
through
DI08
(Data
Input/Output)
are the eight
data
lines
used
to transmit
both
data
and
multiline commands
and
are
bidirectional. The
data
lines operate
with
low true logic.
Bus
Management
Lines
The five
bus
management lines help to ensure
proper
in-
terface control
and
management. These lines are
used
to
send
the uniline commands.
A
TN
(Attention)
-The
A
TN
line
is
one of the more
im-
portant
management lines
in
that
the state of this line de-
termines
how
information
on
the
data
bus
is
to
be
inter-
preted.
IFC (Interface
Clear)-
As the name implies, the IFC line
controls clearing of instruments from the bus.
REN
(Remote
Enable)-
The REN line
is
used
to place the
instrument
on
the
bus
in
the
remote mode.
EOI
(End
or
Identify)-
The EOI
is
usually
used
to mark
the
end
of a multi-byte
data
transfer sequence.
SRQ (Service
Request) - This line is
used
by
device
when
they require service from the controller.