CCST 45 — Verifying an Alarm Setpoint with Dead Band

Briefing

You are loop-checking a high alarm. A calibrator simulates the process value PV_PCT (0 to 100 percent) at the transmitter. HI_ALARM: on above 80 percent, off below 75 percent, and unchanged between 75 and 80 (inclusive). That 5 percent dead band stops the alarm chattering. ALARM_VERIFIED: the loop check record. It latches only once the alarm has been seen to come ON and then seen to clear. It stays set when the value rises again. RESET_CHECK clears the record, so the check can be repeated: after a reset the alarm has to be seen to come on and clear again before it verifies. Comparisons are built with Compare boxes, or in the Code tab as contacts, for example | (A > B) | := LAMP ;

Objectives

• HI_ALARM trips above 80 percent and clears below 75 percent
• Between 75 and 80 (inclusive) the alarm keeps its state, rising and falling
• ALARM_VERIFIED latches only after the alarm has come on and then cleared
• RESET_CHECK clears the record so the check can be run again

I/O Table

Your program will be tested against:

All test cases run automatically when you submit. Assertions are hidden until you pass.

1. #150 percent → quiet

   Nothing on below the setpoint

2. #285 percent → alarm on, not verified

   Trips above 80

3. #385 then 78 → alarm still on

   Dead band holds the alarm on

4. #485, 78, 74 → cleared and verified

   The record sets once the alarm has come on and cleared

5. #550 then 78 → still quiet

   The dead band does not trip a rising value

6. #6Reset clears the record

   A reset starts the check again from scratch

7. #7Exactly 80 and 75 hold the state

   The band edges are inside the band