A-3
A1
A
Figure A-1 IEEE 488 Bus
Devices connected to the bus may act as talkers, listeners, controllers, or
combinations of the three functions, depending upon their internal capability.
The system controller is a controller that becomes active at power turn-on.
It is the Bus manager and the initial controller-in-charge.
A controller can send interface messages to manage the other devices,
address devices to talk or listen and command specific actions within
devices.
A talker sends device dependent messages, i.e., data, status.
A listener accepts interface messages, bus commands and device-
dependent messages, i.e., setup commands, data.
Bus systems can be as simple as two devices; one a talker always sending
data to a second device which listens to the data. Larger systems can have
one or more controllers and many devices (the IEEE 488 driver specifications
limit the total number of units on one bus system to 15). Only one controller
can be the controller-in-charge at any given time. Control originates with
the system controller and is passed back to other controller(s) as required.
Control can be passes back to the system controller or to another controller
after the completion of the task. The system controller has the capability of
taking control back at any time and resetting all addressed devices to their
unaddressed state.
Each bus device is identified by a five-bit binary address. There are 31
possible primary addresses 0 through 30. Address 31 is reserved as the
'untalk' or 'unlisten' command. Some devices contain sub-functions, or the
DEVICE B
Able to Talk
and to Listen
e.g. DVM
DEVICE C
Able to Listen
e.g. Signal
generator
IFC
ATN
SRQ
REN
EOI
DAV
NRFD
NDAC
DIO1-8
Bus Control
Lines
Byte Transfer
Control Lines
Data Bus
(8 Lines)
DEVICE A
Able to Talk,
Listen and Control
e.g. Computer
