IEEE-488 Bus Overview
D-4
Uniline commands
ATN, IFC and REN are asserted only by the controller. SRQ
is asserted by an external device. EOI may be asserted either
by the controller or other devices depending on the direction
of data transfer. The following is a description of each com-
mand. Each command is sent by setting the corresponding
bus line true.
REN (Remote Enable) — REN is sent to set up instruments
on the bus for remote operation. When REN is true, devices
will be removed from the local mode. Depending 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 attempt-
ing 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 allowing
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.
Table D-1
IEEE-488 bus command summary
Command type
Command
State of
ATN line
Comments
Uniline
REN (Remote Enable)
EOI
IFC (Interface Clear)
ATN (Attention)
SRQ
X
X
X
Low
X
Set up devices for remote operation.
Marks end of transmission.
Clears interface.
Defines data bus contents.
Controlled by external device.
Multiline
Universal
LLO (Local Lockout)
DCL (Device Clear)
SPE (Serial Enable)
SPD (Serial Poll Disable)
Low
Low
Low
Low
Locks our local operation.
Returns device to default conditions.
Enables serial polling.
Disables serial polling.
Addressed
SDC (Selective Device Clear)
GTL (Go To Local)
Low
Low
Returns unit to default conditions.
Returns device to local.
Unaddressed
UNL (Unlisten)
UNT (Untalk)
Low
Low
Removes all listeners from the bus.
Removes any talkers from the bus.
Common
—
High
Programs IEEE-488.2 compatible instruments for
common operations.
SCPI
—
High
Programs SCPI compatible instruments for particular
operations.
ATN (Attention) — The controller sends ATN while trans-
mitting 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 re-
quire no addressing. All devices equipped to implement such
commands will do so simultaneously when the commands
are transmitted. As with all multiline commands, these com-
mands are transmitted with ATN true.
LLO (Local Lockout) — LLO is sent to the instrument to
lock out the LOCAL key and thus all their front panel con-
trols.
DCL (Device Clear) — DCL is used to return instruments to
some default state. Usually, instruments return to their pow-
er-up conditions.
