Keithley 2002 User Manual page 307

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.
Addressed multiline commands
Addressed commands are multiline commands that must be
preceded by the device listen address before that instrument
will respond to the command in question. Note that only the
addressed device will respond to these commands. Both the
commands and the address preceding it are sent with ATN
true.
SDC (Selective Device Clear) — The SDC command per-
forms essentially the same function as the DCL command
except that only the addressed device responds. Generally,
instruments return to their power-up default conditions when
responding to the SDC command.
GTL (Go To Local) — The GTL command is used to remove
instruments from the remote mode. With some instruments,
GTL also unlocks front panel controls if they were previ-
ously locked out with the LLO command.
GET (Group Execute Trigger) — The GET command is used
to trigger devices to perform a specific action that depends
on device configuration (for example, take a reading).
Although GET is an addressed command, many devices
respond to GET without addressing.
Address commands
Addressed commands include two primary command groups
and a secondary address group. ATN is true when these com-
mands are asserted. The commands include:
LAG (Listen Address Group) — These listen commands are
derived from an instrument's primary address and are used to
address devices to listen. The actual command byte is
obtained by ORing the primary address with $20.
TAG (Talk Address Group) — The talk commands are
derived from the primary address by ORing the address with
$40. Talk commands are used to address devices to talk.
SCG (Secondary Command Group) — Commands in this
group provide additional addressing capabilities. Many
devices (including the Model 2002) do not use these
commands.
Unaddress commands
The two unaddress commands are used by the controller to
remove any talkers or listeners from the bus. ATN is true
when these commands are asserted.
UNL (Unlisten) — Listeners are placed in the listener idle
state by the UNL command.
UNT (Untalk) — Any previously commanded talkers will be
placed in the talker idle state by the UNT command.
Common commands
Common commands are commands that are common to all
devices on the bus. These commands are designated and
defined by the IEEE-488.2 standard.
Generally, these commands are sent as one or more ASCII
characters that tell the device to perform a common
operation, such as reset. The IEEE-488 bus treats these
commands as data in that ATN is false when the commands
are transmitted.
SCPI commands
SCPI commands are commands that are particular to each
device on the bus. These commands are designated by the
instrument manufacturer, and are based on the instrument
model defined by the Standard Commands for Programma-
ble Instruments (SCPI) Consortium's SCPI standard.
Generally, these commands are sent as one or more ASCII
characters that tell the device to perform a particular opera-
tion, such as setting a range or closing a relay. The IEEE-488
bus treats these commands as data in that ATN is false when
the commands are transmitted.
Command codes
Command codes for the various commands that use the data
lines are summarized in Figure D-3. Hexadecimal and the
decimal values for the various commands are listed in Table
D-2.
IEEE-488 Bus Overview
D-5