Message Concepts - HP 7470A Interfacing And Programming Manual

Listeners — devices which receive information sent on the bus
when they have been addressed.
Controllers — devices that can specify the talker and listeners for
an information transfer. Controllers can be categorized as one of two
types:
0 Active Controller — the current controlling device on the bus.
Only one device can be the active controller at any time.
0 System Controller — the only controller that can take priority
control of the bus if it is not the current active controller. Although
each bus system can have only one system controller, the system
can have any number of devices capable of being the active
controller.
A typical HP-IB system is shown below.
VOLTAGE
SOURCE SVSTEM
CONTROLLER
Lgéé/ER <j- HP-|B —-—> CONTROLLER
CB
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0. Lu
I l­
o 2
DIGITAL E "'
VOLT~ PRINTER - PLOTTER
METER
Message Concepts
Devices which communicate along the interface bus are transferring
quantities of information. The transfer of information can be from one
device to another device, or from one device to more than one device.
These quantities of information can easily be thought of as "messages."
In turn, the messages can be classified into 12 types. The list below
gives the 12 message types for the HP-IB.
1.
The Data Message. This is the actual information which is sent
from one talker to one or more listeners along the interface bus.
The Trigger Message. This message causes the listening device(s)
to perform a device—dependentaction when addressed.
. The Clear Message. This message causes either the listening de­
vice(s) or all of the devices on the bus to return to their predefined
device—dependentstates.
A-2 AN HP—IB OVERVIEW