Scale a 4–20 mA Pressure Transmitter

Briefing

A pressure transmitter wired on the 4–20 mA loop returns raw ADC counts in the range 4 000–20 000. The PLC scales this to engineering units using the formula: **PSI = (PT_RAW − 4 000) ÷ 16 000 × 150** So 4 000 counts = 0 PSI, 12 000 counts = 75 PSI, 20 000 counts = 150 PSI. The physics engine performs this calculation and publishes a ready-to-use BOOL signal — **PUMP_OK_PRESSURE** (true when PSI < 120 and the transmitter is healthy). Your job is to write the supervisory logic: 1. Latch a **RUN_BIT** on START, drop on STOP. 2. Gate **PUMP_PERMIT** on RUN_BIT AND PUMP_OK_PRESSURE — the pump may only run when pressure is safe. 3. Latch **PRESSURE_ALARM** when running and PUMP_OK_PRESSURE drops (pressure too high). 4. Drive **FAULT_LAMP** from the **TRANSMITTER_FAULT** signal (open-circuit 4–20 mA loop: counts below 3 600). The 4–20 mA standard means a broken wire reads near 0 mA, not 0 PSI — the fault band below 3.6 mA (3 600 counts) is how you distinguish "zero pressure" from "broken transmitter".

Objectives

• Latch RUN_BIT: SET on START, RESET on STOP
• PUMP_PERMIT: ON when RUN_BIT AND PUMP_OK_PRESSURE (PSI < 120, no fault)
• PRESSURE_ALARM latches when running and PUMP_OK_PRESSURE is false
• FAULT_LAMP mirrors TRANSMITTER_FAULT (open-circuit detection)

I/O Table

Your program will be tested against:

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

1. #1START with normal pressure → PUMP_PERMIT on

   PT_RAW = 12000 (75 PSI, PUMP_OK_PRESSURE = true) + START → PUMP_PERMIT on within one scan

2. #2High pressure (PT_RAW = 19200 ≥ 120 PSI) blocks PUMP_PERMIT

   PT_RAW = 19200 (≈ 143 PSI) → PUMP_OK_PRESSURE = false → PUMP_PERMIT must not energise on START

3. #3Pressure rises above 120 PSI while running → PRESSURE_ALARM latches

   Start with normal pressure, then raise PT_RAW above threshold → PRESSURE_ALARM latches and PUMP_PERMIT drops

4. #4Open-circuit (PT_RAW < 3600) → FAULT_LAMP on

   Simulating a broken 4–20 mA wire: counts drop below 3600 → TRANSMITTER_FAULT → FAULT_LAMP on