Keithley 6512 Instruction Manual page 139

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 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:
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 previous-
ly 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). Al-
though GET is an addressed command, some devices may
respond to GET without addressing.
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.
Device-dependent commands
The meaning of the device-dependent commands will depend
on the configuration of the instrument. Generally, these com-
mands are sent as one or more ASCII characters that tell the de-
vice to perform a specific function. For example, the command
sequence F0X is used to place the Model 6512 in the volts
mode. The IEEE-488 bus actually treats these commands as
data in that ATN is false when the commands are transmitted.
A.5 Command codes
Each multiline command is given a unique code that is trans-
mitted over the bus as 7 bit ASCII data. This section briefly ex-
plains the code groups, which are summarized in Figure A-3.
Addressed Command Group (ACG)—Addressed commands
and corresponding ASCII codes are listed in columns 0(A)
and 0(B).
Universal Command Group (UCG)—Universal commands
and values are listed in columns 1(A) and 1(B).
Listen Address Group (LAG)—Columns 2(A) through 3(B)
list codes for commands in this address group. For example,
if the primary address of the instrument is 27, the LAG byte
will be an ASCII left bracket.
Talk Address Group (TAG)—TAG primary address values
and corresponding ASCII characters are listed in columns
4(A) through 5(B).
The preceding address groups are combined together to form
the Primary Command Group (PCG). The bus also has an-
other group of commands, called the Secondary Command
Group (SCG). These are listed in Figure A-3 for information-
al purposes only; the Model 6512 does not have secondary
addressing capabilities.
Note that these commands are normally transmitted with the
7 bit code listed in Figure A-3. For many devices, the condi-
tion of DIO8 is unimportant. However, many devices may re-
quire that DIO8 has a value of logic 0 (high) to properly send
commands.
IEEE-488 Bus Overview
A-5