There are two categories of controllers: system controller and basic controller. Both are able
to control other instruments, but only the system controller has the absolute authority in the
system. In a system with more than one controller, only one controller may be active at any given
time. Certain protocol is used to pass control from one controller to another.
The IEEE-488 bus is limited to 15 devices, including the controller. Thus, any number of
talkers and listeners up to that limit may be present on the bus at one time. Although several
devices may be commanded to listen simultaneously, the bus can have only one active talker, or
communications would be scrambled.
A device is placed in the talk or listen state by sending an appropriate talk or listen command.
These talk and listen commands are derived from an instrument’s primary address. The primary
DEVICE 1
ABLE TO
TALK, LISTEN
AND CONTROL
(COMPUTER)
DEVICE 3
ONLY ABLE
TO LISTEN
(PRINTER)
DEVICE 4
ONLY ABLE
TO TALK
DAV
NRFD
NDAC
IFC
ATN
SRQ
REN
EOI
D IO1 ... 8 DATA
(8 LINES)
HANDSHAKE
BUS
MANAGEMENT
DATA BUS
DATA BYTE
TRANSFER
CONTROL
GENERAL
INTERFACE
MANAGEMENT
DEVICE 2
ABLE TO
TALK AND
LISTEN
7001
TO OTHER DEVICES
gure
-
EEE-488 bus
configuration
IEEE-488 Bus Overview E-3
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