PLC Simulator
Basic
10 min

Fix 06 — Latch won’t release

fix-itfaultlatchresetset-resettripheater

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Fix 06 — Latch won’t release

Briefing

An oven heater runs while there is no trip. If OVER_TEMP is seen, even for an instant, the heater is cut and TRIP_LAMP stays on until someone presses RESET_PB. The reset must be refused while the oven is still too hot. The trip works, but it can never be released. Find out why, and fix it.

Objectives

  • With no fault, the heater runs and the trip lamp is off
  • A brief OVER_TEMP trips the heater and the trip is remembered
  • RESET_PB is refused while OVER_TEMP is still on
  • RESET_PB releases the trip once OVER_TEMP is off, and the heater runs again

Hints

  • Trip the oven, let it cool (OVER_TEMP off), then press RESET_PB. What would have to be true for the trip to release? Compare that with what is true at that moment.
  • The reset condition and the cause of the trip are related. Think about whether the reset should work while the oven is hot, and while it is cool.
  • A trip that is released while the cause is still present is set again straight away. A reset must therefore wait for the cause to be gone.
  • Check a second trip after a reset: a released trip must be able to trip again.

I/O Table

Inputs

OVER_TEMP

Oven over-temperature switch (TRUE = too hot)

BOOL · %I0.0

RESET_PB

Trip reset push-button (TRUE = pressed)

BOOL · %I0.1

Outputs

HEATER

Oven heater contactor

BOOL · %Q0.0

TRIP_LAMP

Trip lamp (stays on until reset)

BOOL · %Q0.1

Your program will be tested against:

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

  1. #1No fault → heater on, trip lamp off

    The heater runs whenever there is no trip

  2. #2Brief OVER_TEMP → heater off, trip lamp stays on

    An instant of overheat is remembered after it has gone

  3. #3RESET while OVER_TEMP is still on → trip stays on

    The reset is refused while the cause is present, and the lamp does not flicker off

  4. #4RESET after cooling → trip lamp off, heater on

    With OVER_TEMP off, RESET releases the trip

  5. #5Overheat again after a reset → trips again

    A released trip can trip a second time

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