User`s manual

186 Chapter 6
O:\Manuals\E6380A_Progguid\Book\Advoper.fm
Advanced Operations
Passing Control
Passing Control
Communications on the HP-IB bus are accomplished according to a
precisely defined set of rules (IEEE 488.1 and 488.2 Standards).
Communication (data transfer) is accomplished by designating one
device to be a talker (source of data or commands) and designating one
or more devices to be listeners (receivers of data or commands). The
device on the bus responsible for designating talkers and listeners is
the Controller.
The structure of the HP-IB bus allows for more than one Controller to
be connected to the bus at the same time. As a means of ensuring that
orderly communications can be established on power-up or when
communications have failed, the rules state that only one Controller
can unconditionally demand control of the bus (through the IFC line).
This controller is referred to as the System Controller. There can be
only one System Controller connected to the bus at any time.
As a means of ensuring orderly communications in environments where
more than one controller is connected to the bus, the rules state that
only one Controller can be actively addressing talkers and listeners at
any given time. This device is referred to as the Active Controller. The
System Controller is the default Active Controller on power-up or after
a bus reset. Controllers which are not the Active Controller are referred
to as Non-Active Controllers. The Active Controller can pass control of
device addressing to one of the Non-Active Controllers. Additionally,
Non-Active Controllers can request control from the Active Controller.
The process by which the Active Controller passes device addressing
responsibility to a Non-Active Controller is referred to as Passing
Control. The Active Controller must first address the prospective new
Active Controller to Talk, after which it sends the Take Control Talker
(TCT) message across the bus. If the other Controller accepts the
message it assumes the role of Active Controller and the previous
Active Controller becomes a Non-Active Controller.
The Test Set has bus control capability (Active/Non-Active Controller).
Additionally the Test Set can be also be configured as the System
Controller. By definition then, the Test Set has the capability to
demand control, pass control, accept control, and request control of the
bus depending upon its configuration, its current operating mode, and
the system configuration. Many possibilities for passing control among
several controllers on the same bus exist. Attempting to identify all the
possible techniques of passing control in such a system is beyond the
scope of this document (refer to the IEEE 488.1 and 488.2 Standards for
additional information).