Maple Systems OIT Family - Manual (Page 27)

Reset
The Reset status coil is set by the OIT each time it powers up or performs a
reinitialization due to exiting the OIT’s local setup menu. This can be used by the PLC to
indicate that the OIT has performed a reset. The PLC can clear the coil and decide what
to do with the information. Since the OIT may have experienced a power down and will
not know the current state of the system, we suggest that the PLC reset all I/O to a known
neutral state before clearing this coil and continuing. For more information on the OIT’s
power up sequence, refer to the OIT Power Up section in CHAPTER 1: Getting Started.
Alarm Stack Full
The Alarm Stack Full status coil is set by the OIT when the OIT’s internal alarm stack is
full. This feature allows the PLC to know that, although it may have sent several alarms
to the OIT, nobody is acknowledging them. Additional alarms sent to the OIT cause the
oldest alarm to be eliminated from the alarm stack. Typically, this coil is not set, but in
the event that it is, a real problem exists: alarms are not being acknowledged by the
operator, either because the alert was not heard or it is too short for an operator to
recognize it. This coil allows the PLC to detect this problem and either issue a higher
priority alarm that has a continuous alert tone or shut down critical parts of the system in
case a hazardous potential exists. When the alarm stack is no longer full, the OIT clears
this coil. For more information on alarms, refer to the Alarm Screens section in
CHAPTER 3: Stored Screens.
Message Stack Full
The Message Stack Full status coil is set by the OIT when the screen stack feature is
enabled and the OIT’s internal screen stack is full. This feature allows the PLC to know
that, although it may have sent several screens to the OIT, nobody has read them yet.
Therefore, this coil allows the PLC to take some sort of corrective action (such as sending
an alarm, or not sending any more screens until this coil is cleared by the OIT). When the
screen stack is no longer full, the OIT clears this coil. For more information on the screen
stack, refer to the Screen Stack section in CHAPTER 5: Displaying Stored Screens.
Message Stack Empty
The Message Stack Empty status coil is set by the OIT when the screen stack feature is
enabled and the OIT’s internal screen stack is empty. This feature could allow the PLC to
send a burst of screen requests to the OIT, wait for the coil to be set, then send another set
of screen requests. This offloads some of the overhead required by the PLC for
interfacing with the operator. When the screen stack is no longer empty, the OIT clears
this coil. For more information on the screen stack, refer to the Screen Stack section in
CHAPTER 5: Displaying Stored Screens.
Clear Alarm
The Clear Alarm status coil is monitored by the OIT (every 10 seconds) to allow the PLC
to clear the currently displayed alarm. If this coil is set by the PLC, the OIT cancels any
alarm that is in progress, and continues normal operation. Then the OIT clears this coil to
indicate to the PLC that the action has been carried out. This feature can be used to allow
the PLC to cancel an alarm if the condition which caused the alarm no longer exists.
However, care should be exercised when using this feature if multiple alarms are in the
stack, as the PLC does not know which alarm it is clearing unless the CMR is used. For
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