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Appendix D: IEEE-488 bus overview
Addressed multiline commands
Addressed commands are multiline commands that must be preceded by the device listen address
before that instrument responds to the command in question. Note that only the addressed device
responds to these commands. Both the commands and the address preceding it are sent with ATN
true.
SDC (selective device clear): The SDC command performs 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 previously 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.
Addressed commands
Addressed commands include two primary command groups and a secondary address group. ATN is
true when these commands 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 6517B) 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 are 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 because ATN
is false when the commands are transmitted.
D-8
Model 6517B Electrometer Reference Manual
6517B-901-01 Rev. C / August 2015
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