Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 35)

Section 3 IEEE Operating Modes
3.1
IEEE Operating Modes
3.1 Introduction
There are four types of IEEE bus devices: Active Controllers, Peripherals, Talk-
only devices, and Listen-always devices. Talk-only and Listen-always devices are
usually used together, in simple systems, such as a Talk-only digitizer sending results
to a Listen-always plotter. In these simple systems no controller is needed because the
talker assumes that it is the only talker on the bus, and the listener(s) assume that they
are all supposed to receive all the data send over the bus. This is a simple and effective
method of transferring data from one device and another, but is not adequate for more
complex systems where, for example, one computer is controlling many different bus
devices.
In more complex systems, the Active Controller sends commands to the various
bus Peripherals telling them what to do. Commands such as Unlisten, Listen Address
Group, Untalk, and Talk Address Group are sent by the controller to specify which
device is to send data, and which are to receive it. For more details about the IEEE bus
protocols see the section 'IEEE 488 Primer'.
When an IEEE bus system is first turned on, some device must be the Active
Controller. This device is the System Controller and always keeps some control of the
bus. In particular, the System Controller controls the Interface Clear (IFC) and Remote
Enable (REN) bus management lines. By asserting Interface Clear, the System
Controller forces all the other bus devices to stop their bus operations, and regains
control as the Active Controller.
3.2 Operating Mode Transitions
The System Controller is initially the Active Controller. It can, if desired, Pass
Control to another device and thereby make that device the Active Controller. Note
that the System Controller remains the System Controller, even when it is not the
Active Controller. Of course, the device to which control is passed must be capable of
taking on the role of Active Controller. It would make no sense to try to pass control to
a printer. Control should only be passed to other computers that are capable, and
ready, to become the Active Controller. Further, note that there must be exactly one
System Controller on the IEEE bus. All other potential controllers must be configured
as Peripherals when they power up.