Maple Systems OIT Family - Manual (Page 28)

more information on the CMR, refer to the Current Message Register section in this
chapter. For more information on alarms, refer to the Alarm Screens section in
CHAPTER 3: Stored Screens.
Clear Alarm Stack
The Clear Alarm Stack status coil is similar to the Clear Alarm status coil except that this
coil allows the PLC to clear the entire stack of alarms, including the currently displayed
one. This feature could be used to cancel all existing alarms on the OIT if the PLC
performs a system-wide reset. For more information on alarms, refer to the Alarm
Screens section in CHAPTER 3: Stored Screens.
NOTE:Any alarms in the stack are automatically cleared whenever the OIT is reinitialized.
OIT Pulse
The OIT Pulse status coil is pulsed by the OIT every 10 seconds. This allows the PLC to
determine if the OIT is still communicating with the PLC.
Update PLC Clock
The Update PLC Clock status coil is set by the OIT when it starts sending new time and
date information to the PLC and cleared by the OIT when it finishes. To ensure that the
time and date are not currently being updated, the PLC should verify that this coil is not
set before using the time and date information.
Alarm Stack Empty
The Alarm Stack Empty status coil is set by the OIT when the OIT initializes and when
the OIT’s internal alarm stack is empty. When an alarm is received, the OIT clears this
coil. This feature allows the PLC to determine if the plant floor operator has
acknowledged all currently active alarms. Once all alarms have been acknowledged, the
PLC may go into a “normal” operating state. For more information on alarms, refer to the
Alarm Screens section in CHAPTER 3: Stored Screens.
Acknowledge Alarm
The Acknowledge Alarm status coil is set by the OIT when the operator presses the
ALARM ACK key to clear an alarm. This can be used to signal to the PLC that normal
operation can begin again. It is the PLC’s responsibility to clear this coil once it has been
set by the OIT. For more information on alarms, refer to the Alarm Screens section in
CHAPTER 3: Stored Screens.
Status LED Coils
The Status LED coils are a block of discrete coils in the PLC that are used to control the
state of each status LED (OIT3250, OIT4450, and OIT5400 only). If the corresponding
coil is set, the LED is on. If the corresponding coil is clear, the LED is off. The Status
LEDs allows the OIT operator to continuously monitor eight PLC coils without having to
use the screen. For more information on LEDs, refer to CHAPTER 8: Using the LEDs.
Operational Overview 28
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