CCST 41 — 4-20 mA Scaling with a Live-Zero Check

Briefing

A pressure transmitter sends a 4-20 mA signal. The analog card gives LOOP_MA_X10, the current in tenths of a milliamp, so 4.0 mA reads 40 and 20.0 mA reads 200. Two jobs: 1. PV_PCT is the process variable as a percentage of range: 4 mA is 0 percent and 20 mA is 100 percent. A result within 1 of the true value is accepted. 2. SIGNAL_FAULT is on when the current falls outside the live-zero window: below 3.6 mA (36) or above 21.0 mA (210). The signal never goes below 4 mA when the loop is healthy, so a reading near 0 is a broken wire, not a zero process value. Whole-number maths keeps no fraction, so multiply before you divide. The in-between product is bigger than a small integer; use a DINT for it. Maths is built with Math boxes (ADD, SUB, MUL and DIV), or in the Code tab one assignment per line. Comparisons are built with Compare boxes, or in the Code tab as contacts, for example | (A > B) | := LAMP ;

Objectives

• 4.0 mA (40) gives 0 percent, 12.0 mA (120) gives 50 and 20.0 mA (200) gives 100
• A current between the edges of the window leaves SIGNAL_FAULT off
• Below 3.6 mA (36) or above 21.0 mA (210) turns SIGNAL_FAULT on

I/O Table

Your program will be tested against:

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

1. #14.0 mA → 0 percent

   The bottom of the signal is 0 percent

2. #28.0 mA → 25 percent

   A quarter of the 16 mA span

3. #312.0 mA → 50 percent

   Mid-scale

4. #420.0 mA → 100 percent

   Top of the signal

5. #5Healthy signal → no fault

   No fault anywhere inside the window

6. #60.0 and 2.0 mA → fault

   A dead or broken loop is flagged

7. #723.0 mA → fault

   A shorted loop or saturated transmitter is flagged

8. #810.0 mA → about 37.5 percent

   A reading that does not divide evenly

9. #9Window edges: 3.6 and 21.0 mA are healthy

   Exactly on the edge is not a fault; one tenth outside is