DESCRIPTION AND OPERATION
SECURITY FUNCTIONS
I-E96-207A
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SECURITY FUNCTIONS
Hardware and software security functions help minimize the
impact of errors on the controller process. There are two types
of security: module security and control input security.
Module Security
Module security is provided by a machine fault timer (MFT).
The MFT is reset periodically by the processor. If an error is
detected, the MFT is not reset and:
•
Execution of algorithms ceases.
•
Analog outputs go to their user-selected default states.
•
Digital outputs de-energize.
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NVRAM is inhibited (cannot be erased or written).
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Module Bus and Station communications cease.
•
The Status LED goes solid RED.
Control Input Security
All input points to the controller module have either a GOOD or
BAD quality status. GOOD quality status is the normal opera-
tion of the controller module with the parameters of the system.
BAD quality status results from an out-of-range signal or a sig-
nal not getting through from the module bus or communication
highway. The function block TEST QUALITY (function code 31)
is used to test the input quality. Depending on the importance of
the input, the TEST QUALITY block can be configured to trans-
fer the associated operator manual-auto station to manual, shut
down the process, or send a warning to an annunciator.
Internal Software Security Functions
Two functions are performed by the internal software: module
diagnostics and module status check. Module diagnostics are
performed when the controller is powered up. If a problem is
detected the module stops immediately and the status LED
turns red. As a background idle task, the module status check
constantly verifies ROM and NVRAM checksums. If a discrep-
ancy is found in any checksum the module stops immediately
and the status LED turns red.
Control Software Security
The control software is responsible for local I/O problems,
remote I/O problems, station problems and redundancy
errors.
Local and remote I/O errors cause the controller to assign a
bad status to the input signals. Local errors occur when:
•
An I/O signal or voltage reference is out of range.
