Keithley 708A Instruction Manual Download

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IEEE-488 Bus Overview

C-5

C.4.2 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 com-
mands 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 all their front panel
controls.

• DCL (Device Clear) — DCL is used to return instru-

ments to some default state. Instruments usually return
to their power-up conditions.

• SPE (Serial Poll Enable) — SPE is the first step in the

serial polling sequence that is used to determine which
device has requested service.

• SPD (Serial Poll Disable) — SPD is used by the con-

troller to remove all devices on the bus from the serial
poll mode and is generally the last command in the
serial polling sequence.

C.4.3 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

performs essentially the same function as the DCL
command except that only the addressed device re-
sponds. Generally, instruments return to their power-up
default conditions when responding to the SDC com-
mand.

• 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.

C.4.4 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 com-

mands 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 708A) do not use
these commands.

C.4.5 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.

C.4.6 Command codes

Command codes for the various commands that use the data
lines are summarized in Figure C-3. Hexadecimal and deci-
mal values for the various commands are listed in Table C-2.

Table C-2

Hexadecimal and decimal command codes

Command

Hex value

Decimal value

GTL
SDC
GET
LLO
DCL
SPE
SPD
LAG
TAG
SCG
UNL
UNT

01
04
08
11
14
18
19
20-3F
40-5F
60-7F
3F
5F

1
4
8
17
20
24
25
32-63
64-95
96-127
63
95

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