29
CHAPTER 4: IEEE Operating Modes
Figure 4-1. IEEE bus operating modes state diagram.
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.
Active
System
Controller
SC-CA
Active
Controller,
Not System
Controller
*SC-CA
System
Controller,
Not Active
SC-*CA
Peripheral,
Not System
Controller
*SC-*CA
Power On
Power On
Controller Active (CA)
Peripheral (*CA)
Not System Controller (*SC)
Passes Control
Receives Control or
Asserts Interface Clear
Receives Control
Passes Control or
Detects Interface Clear
System Controller (SC)
