8800 EU Alarm Processing Notes

As a point is converted from raw values to an engineering units value (EUV) alarms are checked and an alarm level is generated for a point. A point can have three alarm types associated with it: validity, engineering units, and rate of change. The following documents when the different alarm types are checked and what precedence of alarms this generates.

Analog Input

Analog input points can have all three alarm types

  1. Raw value is scanned. If it cannot be scanned, the EU scan exception (XS) alarm level is generated. No more conversion can be performed on this point. The exception quality bit (X) is set for this point and the old EUV is maintained as the current EUV for this point since it is impossible to calculate a new one. Skip to step 6.
  2. The raw value is gain compensated. This converts the raw value from counts to the units required by the EU conversion (usually volts). At this time, the validity limits are checked. If the point has violated a validity limit this becomes the validity alarm limit, the exception quality bit (X) is set and the conversion continues.
  3. The gain compensated value is now converted to a new EUV. If the conversion fails due to a mathematical exception (such as square root of a negative number), the EU alarm level of the point becomes mathematical exception (XM), the exception quality bit (X) is set and the old EUV is maintained as the current EUV for this point (since it was impossible to calculate a new one).
  4. If the point does not have a scan or mathematical exception, EU alarm processing continues. The EUV is compared against the EU alarm limits and return deadbands to see what alarm level the point is now in.
  5. The rate of change alarms are next checked. Currently rate of change alarming has not been implemented since a meeting is required to determine the implementation details of rate of change processing in a significant change processing philosophy.
  6. At this point the alarm levels of the point are well known along with the quality code’s exception bit and the EUV.

After step 6, an alarm minimization algorithm is executed to minimize alarm ‘ringing’. This algorithm and its effect on the alarm level given to the system are not discussed here.

Analog Compute

Analog computed points can have EU and rate of change alarm types.

  1. The compute task receives a new value for a point used by a computed point.
  2. The computed point’s EUV is computed. If the conversion fails due to a mathematical exception (such as divide by zero), the EU alarm level of the point becomes mathematical exception (XM), the exception quality bit (X) is set and the old EUV is maintained as the current EUV for this point (since it was impossible to calculate a new one).
  3. If the point does not have the exception bit set, EU alarm processing continues. The EUV is compared against the EU alarm limits and return deadbands to see what alarm level the point is now in.
  4. The rate of change alarms are next checked. Currently rate of change alarming has not been implemented since a meeting is required to determine the implementation details of rate of change processing in a significant change processing philosophy
  5. At this point the alarm level of the point is well known along with the quality codes exception bit and the EUV.

After step 5, an alarm minimization algorithm is executed to minimize alarm ‘ringing’. This algorithm and its effect on the alarm level given to the system are not discussed here.

Digital Input

Only an EU alarm type exists for a digital input.

  1. Raw value is scanned. If it cannot be scanned, the exception quality bit (X) is set for this point and the old EUV is maintained as the current EUV for this point since it is impossible to calculate a new one. Skip to step 3.
  2. The digital is converted to a new EUV and the EU alarm state is set according the requirements for that states alarm level.
  3. At this point the alarm level of the point is well known along with the quality codes exception bit and the EUV.

After step 3, an alarm minimization algorithm is executed to minimize alarm ‘ringing’. This algorithm and its effect on the alarm level given to the system are not discussed here.

Digital Compute

Only an EU alarm type exists for a digital computed.

  1. The compute task receives a new value for a point used by a computed point.
  2. The digital is converted to a new EUV and the EU alarm state is set according the requirements for that states alarm level.
  3. At this point the alarm level of the point is well known along with the quality codes exception bit and the EUV.
  4. After step 3, an alarm minimization algorithm is executed to minimize alarm ‘ringing’. This algorithm and its effect on the alarm level given to the system are not discussed here.

NOTE: Preliminary data. Not official.