Keithley 2000, User Manual, Page: 263 / 280

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 command. 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 con 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.
ATN (Attention) — The controller sends ATN while transmitting addresses or multiline com-
mands.
SRQ (Service Request) — SRQ is asserted by a device when it requires service from a con-
troller.
Universal multiline commands
Universal commands are those multiline commands that require no addressing. All devices
equipped to implement such commands will do so simultaneously when the commands are
transmitted. As with all multiline commands, these commands 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 controls.
DCL (Device Clear) — DCL is used to return instruments to some default state. Usually, in-
struments return to their power-up conditions.
SPE (Serial Poll Enable) — SPE is the first step in the serial polling sequence which is used
to determine which device has requested service.
SPD (Serial Poll Disable) — SPD is used by the controller to remove all devices on the bus
from the serial poll mode and is generally the last command in the serial polling sequence.
IEEE-488 Bus Overview E-9