Chapter 4. CPU Operation
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April 2018
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Intelligent Option Module Self-Test Completion
Intelligent option modules may take a longer time to complete their self-tests than the CPU due to the
time required to test communications media or other interface devices. As an intelligent option module
completes its initial self-tests, it tells the CPU the time required to complete the remainder of these self-
tests. During this time, the CPU provides whatever additional information the module needs to complete
its self-configuration, and the module continues self-tests and configuration. If the module does not
report back in the time it specified, the CPU marks the module as faulted and makes an entry in one of
the fault tables. When all self-tests are complete, the CPU obtains reports from the module as generated
during that particular module’s power
-up self-test and places fault information (if any) in the fault tables.
Intelligent Option Module Dual Port Interface Tests
After completion of the intelligent option module self-test and results reporting, integrity tests are jointly
performed on the dual-port interface used by the CPU and intelligent option module for communications.
These tests validate that the two modules are able to pass information back and forth, as well as verify
the interrupt and semaphore capabilities needed by the communications protocol. After dual port
interface tests are complete, the communications messaging system is initialized.
I/O System Initialization
If the module is an input module, no further configuration is required. If the module is an output module,
the module is commanded to go to its default state. The output modules default to all outputs off at
power-up and in failure mode, unless configured otherwise.
A bus transmitter module is interrogated about what expansion racks are present in the system. Based
on the bus transmitter module’s response, the CPU adds those racks and their associated slots into the
list of slots to be configured.
Finally, the I/O Scanner performs its initialization. The I/O Scanner initializes all the I/O controllers in the
system by establishing the I/O connections to each I/O bus on the I/O controller and obtaining all I/O
configuration data from that I/O controller. This configuration data is compared with the stored I/O
configuration and any differences reported in the I/O Fault Table. The I/O Scanner then sends each I/O
controller a list of the I/O modules to be configured on the I/O bus. After the I/O controllers have been
initialized, the I/O Scanner replaces the factory default settings in all I/O modules with any application-
specified settings.
4.11.2
Power-Down Sequence
System power-down occurs when the power supply detects that incoming power has dropped for more
than 15ms.
