Tektronix 2714, Programmer'S Manual, Page: 281 / 308

Appendix B: GPIB System Concepts
2714 & 2715 Programmer Manual
B–5
Talkers, Listeners, and Controllers
A talker is an instrument that can send messages and data over the bus. A listener
is an instrument that can accept messages and data from the bus. An instrument
can be a talker only, listener only, or be both a talker and a listener. Unless a
device is in the talk-only or listen-only mode, it can only communicate with
other devices on the bus when it is enabled to do so by the controller in charge of
the instrumentation system.
A controller is an instrument that determines, by software routines, which
instrument will talk and which instruments will listen during any given time
interval. The controller has the ability to assign itself as a talker or a listener
whenever the program routine requires it. In addition to designating the current
talker and listeners for a particular communication sequence, the controller is
assigned the task of sending special codes and commands (called interface
control messages) to any or all instruments on the bus. A complete operating
system may contain more than one controller. The IEEE standard has provisions
for a system controller that operates with another controller in charge of the bus.
The controller that is in charge of the bus can take control only when it is
directed to do so by the system controller. The system controller may be, but is
not necessarily, the controller in charge of the bus.
Interface Control Messages
The two types of interface control messages are multi-line messages sent over
the data bus and uni-line messages. A message that shares a group of signal lines
with other messages, in some mutually exclusive set, is called a multi-line
message. Only one multi-line message [message byte] can be sent at one time. A
message sent over a single line is called a uni-line message. Two or more of
these messages can be sent concurrently.
Only multi-line messages are discussed here; uni-line messages are discussed
later in this section. See GPIB Signal Line Definitions.
The interface control messages (Table B–4) are sent and received over the data
bus only when the ATN (attention) line is asserted (true). Interface message
coding can be related to the ISO (International Standards Organization) 7-bit
code by relating data bus lines DIO1 through DIO7 to bits B1 through B7,
respectively, in the Bits column in Table B–4.
Interface control messages (Table B–2 and Table B–3) include the primary talk
and listen addresses for instruments on the bus, addressed commands, universal
commands, and secondary addresses for devices interfaced through a primary
instrument. Only instruments previously addressed to listen will respond to
addressed commands. All instruments, whether they have been addressed or not,
will respond to universal commands.