Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 37)

Section 3 IEEE Operating Modes
3.3
The bottom half of the state diagram shows the two operating states of a Not
System Controller device. At power on, it is a Peripheral to the System Controller
which is the Active Controller. If it receives control from the Active Controller, it
becomes the new Active Controller. Even though it is the Active Controller, it is still
not the System Controller. The System Controller can force the Active Controller to
give up control by asserting Interface Clear. The Active Controller can also give up
control by Passing Control to another device, which may or may not be the System
Controller.
In summary, a bus device is set in hardware as either the sole System Controller
in the system, or as a non-System Controller. At power on, the System Controller is
the Active Controller, and the other devices are Peripherals. The System Controller
can give up control by Passing Control, and can regain control by asserting Interface
Clear, or by receiving control. A Peripheral can become the Active Controller by
receiving control, and can give up control by Passing Control, or upon detecting
Interface Clear.
3.3 System Controller Mode
The most common the Micro488/EX configuration is as the System Controller,
controlling several IEEE bus instruments. In this mode, the Micro488/EX can perform
all of the various IEEE bus protocols necessary control and communicate with any
IEEE 488 bus devices. As the System Controller in the Active Controller mode, the
Micro488/EX can use all of the commands available for the Active Controller state,
plus control the Interface Clear and Remote Enable lines. The allowed bus commands
and their actions are as follows:
ABORT Pulse Interface Clear.
LOCAL Unassert Remote Enable, or send Go To Local to selected
devices.
REMOTE Assert Remote Enable, optionally setting devices to
Remote.
LOCAL LOCKOUT Prevent local (front-panel) control of bus devices.
CLEAR Clear all or selected devices.
TRIGGER Trigger selected devices.