APPENDIX£
IEEE-488
Bus
Overview
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Table
E-1.
IEEE-488 Bus
Command
Summary
Command
Type Command
Uniline REN (Remote Enable)
EOI
IFC
(Interface Clear)
A TN (Attention)
SRQ
Multiline
Universal
LLO
(Local Lockout)
DCL (Device Clear)
SPE
(Serial Enable)
SPD
(Serial Poll Disable)
Addressed
SOC
(Selective Device Clear)
GTL
(Go To Local)
Unaddressed
UNL (Unlisten)
UNT (Untalk)
Device-
dependent
*X=
Don't
Care
scription of each command. Each command
is
sent
by
set-
ting the corresponding bus line true.
REN (Remote Enable) - REN
is
sent to set
up
instru-
ments on the bus for remote operation. When REN
is
true, devices
will
be removed from the local mode. De-
pending
on
device configuration, all front panel controls
except the LOCAL button
(if
the device
is
so equipped)
may be locked
out
when
REN
is
true. Generally, REN
should be sent before attempting to program instruments
over the bus.
EOI
(End
or
Identify)-
EOI
is
used to positively identify
the last byte in a multi-byte transfer sequence, thus al-
lowing data words
of
various lengths to be transmitted
easily.
IFC
(Interface
Clear)-
IFC
is
used to clear the interface
and
return all devices to the talker
and
listener idle states.
State
of
ATN
Line* Comments
X
Sets
up
devices for remote op-
eration.
X
Marks
end
of transmission.
X
Clears interface
Low
Defines data
bus
contents.
X
Controlled
by
external device.
Low Locks
out
local operation.
Low
Returns device to default condi-
tions.
Low
Enables serial polling.
Low Disables serial polling.
Low Returns unit to default condi-
tions.
Low Returns device to local.
Low Removes all listeners from bus.
Low Removes
any
talkers from bus.
High
Programs Model236 /237/238
for various modes.
ATN (Attention)
-The
controller sends ATN while
transmitting addresses
or
multiline commands.
SRQ (Service Request) -
SRQ
is
asserted
by
a device
when
it
requires service from a controller.
Universal Multiline Commands
Universal commands are those multiline commands that
require no addressing. All devices equipped to imple-
ment such commands
will
do
so simultaneously
when
the commands
is
transmitted. As with all multiline com-
mands, these commands are transmitted with A TN true.
LLO
(Local Lockout) - LLO
is sent to the instrument to
lock
out
their front panel controls.
DCL (Device
Clear)-
DCL
is
used to return instruments
to some default state. Usually, instruments return to their
power-up conditions.
E-4
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