Model 6220/6221 Reference Manual IEEE-488 Bus Overview C-9
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Uniline commands
ATN, IFC, and REN are asserted only by the controller. SRQ is asserted by an
external device. EOI may be asserted either by the controller or other devices
depending 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 instruments on the bus for remote
operation. When REN is true, devices will be removed from the local mode.
Depending on device configuration, all front panel controls, except the LOCAL
button (if the device is so equipped), may be locked out when REN is true. Gener-
ally, REN should be sent before attempting to program instruments over the bus.
EOI (End or Identify) — EOI is used to positively identify 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 interface and return all devices to
the talker and listener idle states.
ATN (Attention) — The controller sends ATN while transmitting addresses or mul-
tiline 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 implement such commands will do so simultaneously
when the commands are transmitted. As with all multiline commands, these com-
mands 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 instruments 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
which is used to determine which device has requested service.
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.
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