CCST 46 — Loop Wiring and Polarity Fault from Readings

Briefing

A 4-20 mA loop is dead at commissioning. You measure two things: LOOP_MA_X100, the loop current at the PLC input in hundredths of a milliamp (1200 = 12.00 mA), and TX_VOLTS_X10, the DC voltage across the transmitter terminals in tenths of a volt (240 = +24.0 V, negative means the meter leads read the other way). Name the fault. Exactly one lamp is on: - LOOP_HEALTHY: current from 3.60 mA (360) to 21.00 mA (2100). - OVER_RANGE: current above 21.00 mA. A shorted loop or a transmitter driving high. - With almost no current (under 1.00 mA, which is 100), the voltage across the transmitter tells the cause: - OPEN_CIRCUIT: the supply is there (above +5.0 V, which is 50) but no current flows. - NO_SUPPLY: no voltage either, within +/-5.0 V (-50 to 50). - REVERSED_POLARITY: a negative voltage below -5.0 V (-50). Currents from 1.00 to 3.59 mA are not part of this check. Comparisons are built with Compare boxes, or in the Code tab as contacts, for example | (A > B) | := LAMP ; A comparison cannot be written against a negative number, such as | (A > -5) |. To test a negative reading, flip its sign first with SUB(0, X) and compare the flipped value with a positive limit.

Objectives

• A current from 3.60 to 21.00 mA lights only LOOP_HEALTHY, whatever the voltage
• Above 21.00 mA lights only OVER_RANGE
• Under 1.00 mA the voltage decides: supply present is OPEN_CIRCUIT, none is NO_SUPPLY, negative is REVERSED_POLARITY

I/O Table

Your program will be tested against:

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

1. #112.00 mA, 24 V → healthy

   A normal live signal

2. #20 mA, +24 V → open circuit

   Supply present, no current

3. #30 mA, 0 V → no supply

   Nothing feeds the loop

4. #40 mA, -24 V → reversed polarity

   Wired backwards

5. #523.50 mA → over range

   Short or transmitter driving high

6. #6Healthy at any voltage; one lamp at a time

   The healthy verdict is judged on current alone

7. #7Window edges

   Each edge belongs to exactly one verdict