Keithley 486 Instruction Manual page 226

APPENDLXE
IEEE-488 Bus 0vm.m
SPE (Serial Poll Enable) -SPE is the first step in the serial
polling sequence which is used to deter&e which de-
vice has requested service.
SPD (Serial Poll Disable) - SPD is used by the controller
to rexnove all devices on the bus from the serial poll mode
and is generally the last command in the serial polling se-
quence.
Addressed Multiline Commands
Addressed commands are multiline commands that
must be preceded by the device listen address before that
instrument will respond to the con-man d in question.
Notethatonlytheaddresseddevicewillrffpondtothese
commands. Both the commands and the address preced-
ing it are sent with ATN tie.
SDC (Selective Device Clear) -The SDC command per-
forms essentially the same function as the DCL command
except that only the addressed device responds. Gener-
alIy, instrunents return to their power-up default condi-
tions when
responding to the SDC command.
GTL (Go To Local) -The GTL command is used to re-
move instruments from the remote mode. With some in-
struments, GTL also unlocks front panel conhols 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.
Address Commands
Addressed commands include two primary command
groups and a secondary address group. ATN is true
when these commands are asserted. The commands in-
clude:
LAG (Listen Address Group) -These listen commands
are derived from an instnnnent's primary address and
are used to address devices to listen. The actual corn-
mand byte is obtained by OF&g 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 486/487) do not use these
commands.
Unaddress Commands
The two unaddress commands are used by the controller
to renwve any talkers or listeners from the bus. ATN is
true when these commands are asserted.
UNL (Unlisten)--tenersareplacedinthelisteneridle
state by the UNL command.
UhT (untalk) - Any previously commanded talkers
will be placed in the talker idle state by the UNT corn-
mand.
Device-dependent Commands
The purpose of the device-dependent cornman dswillde-
pend on the configuration of the instrument. Generally,
these commands are sent as one or more ASCII characters
that tell the device to perform a specific function.
The IEEE-488 bus actually treats these cornman ds as data
in that ATN is false when the comman ds are transmitted.
Command Codes
Command codes for the various commands that use the
data lines are summarized in Figure E-3. Hexadecimal
and the decimal values for the various commands are
listed in Table E-2.
E-5