IEEE-488 Bus Overview
D-4
byte on the data lines is now valid. NRFD will then go
low, and NDAC will go high once all devices have ac-
cepted the data. Each device will release NDAC at its
own rate, but NDAC will not be released to go high un-
til all devices have accepted the data byte.
The sequence just described is used to transfer both da-
ta, talk and listen addresses, as well as multiline com-
mands. The state of the ATN line determines whether
the data bus contains data, addresses or commands as
described in the following paragraph.˛
Bus Commands
The instrument may be given a number of special bus
commands through the IEEE-488 interface. This sec-
tion briefly describes the purpose of the bus commands
which are grouped into the following three categories.
1. Uniline Commands — Sent by setting the associat-
ed bus lines true. For example, to assert REN (Re-
mote Enable), the REN line would be set low (true).
2. Multiline Commands — General bus commands
which are sent over the data lines with the ATN
line true (low).
3. Common Commands — Commands that are com-
mon to all devices on the bus; sent with ATN high
(false).
4. SCPI Commands — Commands that are particular
to each device on the bus; sent with ATN (false).
These bus commands and their general purpose are
summarized in Table D-1.
Uniline commands
ATN, IFC and REN are asserted only by the controller.
SRQ is asserted by an external device. EOI may be as-
serted either by the controller or other devices depend-
ing on the direction of data transfer. The following is a
description of each command. Each command is sent
by setting the corresponding bus line true.
REN (Remote Enable) — REN is sent to set up instru-
ments on the bus for remote operation. When REN is
true, devices will be removed from the local mode. De-
pending on device configuration, all front panel con-
trols except the LOCAL button (if the device is so
equipped) may be locked out when REN is true. Gen-
erally, REN should be sent before attempting to pro-
gram instruments over the bus.
EOI (End or Identify) — EOI is used to positively iden-
tify the last byte in a multi-byte transfer sequence, thus
allowing data words of various lengths to be transmit-
ted easily.
IFC (Interface Clear) — IFC is used to clear the inter-
face and return all devices to the talker and listener idle
states.
ATN (Attention) — The controller sends ATN while
transmitting addresses or multiline commands.
SRQ (Service Request) — SRQ is asserted by a device
when it requires service from a controller.
Universal multiline commands
Universal commands are those multiline commands
that require no addressing. All devices equipped to im-
plement such commands will do so simultaneously
when the commands are transmitted. As with all mul-
tiline commands, these commands are transmitted
with ATN true.
LLO (Local Lockout) — LLO is sent to the instrument
to lock out the LOCAL key and thus all their front pan-
el controls.
DCL (Device Clear) — DCL is used to return instru-
ments to some default state. Usually, instruments re-
turn to their power-up conditions.
SPE (Serial Poll Enable) — SPE is the first step in the se-
rial polling sequence which is used to determine which
device has requested service.
SPD (Serial Poll Disable) — SPD is used by the control-
ler to remove all devices on the bus from the serial poll
mode and is generally the last command in the serial
polling sequence.
Artisan Scientific - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisan-scientific.com
To Next Page
Loading...
Need help?
General
