Short answer
An analog input does not arrive as a temperature, pressure, or flow value; it arrives as a raw integer count. The configured signal range defines which counts correspond to the transmitter's lower and upper values, while live zero is the nonzero electrical baseline used by ranges such as 4–20 mA. If zero process is mapped to electrical zero instead of live zero, the scaled reading is biased from the start.
Quick reference: Common Industrial PLC Analog Input Raw Count Ranges & Resolution Specifications
| PLC Platform / Hardware Family | Configured Range | Raw Counts @ Min (4 mA / 0 V) | Raw Counts @ Max (20 mA / 10 V) | Counts per mA / Scale Factor |
|---|---|---|---|---|
| Siemens S7-1200 / S7-1500 (Standard AI) | 4–20 mA / 0–10 V | 0 counts (4 mA / 0 V) | 27,648 counts (20 mA / 10 V) | 1,728 counts/mA (Underflow: -4864, Overflow: 32511) |
| Rockwell ControlLogix / CompactLogix (1756-IF8) | 4–20 mA | 4,000 counts (Engineering Units) | 20,000 counts (Engineering Units) | 1,000 counts/mA (High resolution mode: 0 to 32767 counts) |
| Rockwell Micro800 / MicroLogix | 4–20 mA / 0–10 V | 0 counts | 4,095 counts (12-bit) / 65,535 (16-bit) | 256 counts/mA (12-bit) / 4,096 counts/mA (16-bit) |
| Schneider Modicon (M340 / M580) | 4–20 mA | 0 counts (or 4,000 counts) | 10,000 counts (standard factory scaling) | 625 counts/mA |
| Omron CJ2 / NX Series | 4–20 mA | 0 counts | 4,000 counts (1/4000) or 8,000 counts (1/8000) | 250 counts/mA or 500 counts/mA |
| Raw Current Live Zero Configuration Trap | 0–20 mA card on 4–20 mA loop | 5,530 counts (Siemens 4 mA) | 27,648 counts (Siemens 20 mA) | Failure trap: 0 mA reads as 0 counts, hiding broken loop wires |
What to look for
For a linear channel, use engineering value = low EU + (raw − low raw) × (span EU/span raw), with the actual configured endpoints rather than a remembered nominal resolution. Confirm whether diagnostic reserve counts, overrange, underrange, or signed formats are enabled. A correct slope with a fixed offset suggests live-zero or endpoint error; correct endpoints but unstable raw counts suggest reference, shielding, or noise issues.
Verify without exposing personnel to hazardous energy: review input type, channel configuration, scaling constants, and sign convention offline; then apply approved known low, mid, and high test inputs through the prescribed low-energy arrangement. Record raw and scaled values at each point, including alarms. Exact terminal assignments and permissible limits must be checked against the equipment manual.
When the symptom appears in the plant
During commissioning, a water tank level reading in the HMI showed 1500 gallons, but the tank was physically overflowing at its 1000-gallon capacity. The 4-20mA pressure transmitter was correctly set for 0-100 inches of water column. However, the PLC programmer assumed the tank was perfectly cylindrical. In reality, it was a horizontal cylinder, meaning the volume-to-level relationship was non-linear. The linear scaling block was removed and replaced with a strapping table (multipoint curve) to accurately calculate volume based on height.
The 0–20 mA vs 4–20 mA Hardware Channel Configuration Blind Spot
Configuring a PLC analog card for 0–20 mA when connecting a 4–20 mA transmitter creates a severe operational and safety blind spot. At 4.0 mA, a 0–20 mA configured channel reads 20% of full span (e.g. 5,530 counts in Siemens or 3,277 in Rockwell). The programmer scales this offset away in ladder logic, making the HMI look normal during normal operation. However, if a field wire breaks or the transmitter loses power, the signal drops to 0.0 mA. Because the card is configured for 0–20 mA, it reports 0 counts without error. The PLC treats 0 counts as a valid physical measurement (e.g. -25 PSI or -50°C) instead of flagging an under-range loop fault. Always configure the hardware module explicitly for 4–20 mA so the card firmware automatically generates a diagnostic fault bit when loop current drops below 3.6 mA.
Five checks before replacement
- Check the PLC hardware configuration module properties to confirm whether the channel is configured for 4–20 mA or 0–20 mA, unipolar or bipolar.
- Inject a known 12.00 mA (50% span) signal from a calibrated process simulator into the input terminals; verify the raw integer register reads exactly halfway between configured minimum and maximum counts.
- Verify the mathematical scaling formula in ladder logic: Scaled_Value = ((Raw_Input - In_Min) / (In_Max - In_Min)) * (EU_Max - EU_Min) + EU_Min.
- Inject a sub-4 mA signal (3.5 mA) to verify the PLC generates an underrange/wire-break alarm flag rather than calculating negative physical units.
- Confirm whether the process variable requires non-linear characterization (such as square root extraction for differential pressure flow, or strapping tables for horizontal tanks).
Document the raw counts formula directly in PLC rung comments: e.g., 'Siemens 0-27648 counts = 0-150.0 PSI'.
When replacing an obsolete analog module with a newer model, verify A/D conversion resolution; a jump from 12-bit (0-4095) to 16-bit (0-32767) breaks unscaled logic.
Always clamp scaled outputs in PLC code to physical process limits to prevent calculation overflows in downstream PID control blocks.
Bench checklist
- Confirm raw count format: verify whether the input card uses 0–27648 (Siemens), 0–32767 (signed int), or 0–4095 (12-bit).
- Verify live zero mapping: ensure 4.0 mA maps to the correct non-zero raw count (e.g. 0 in 4–20 mA mode, or 5530 in unscaled mode).
- Test the linear slope equation: calculate Expected EU = Low_EU + (Raw - Low_Raw) * Span_EU / Span_Raw.
- Check overrange and underrange limits: confirm how the PLC handles values below 3.6 mA or above 21.0 mA.
- Calibrate with a high-accuracy signal generator at 0%, 25%, 50%, 75%, and 100% of span and log raw counts.
Reference families
- IEC 61131-3 Programming Standards
- ISA-5.1 Instrumentation Symbols & Identification
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- ControlLogix Analog I/O Modules User Manual Rockwell Automation
Official manual explaining resolution, counts, data format, and 4–20 mA scaling.
- FLEX I/O Analog Modules User Manual Rockwell Automation
Official manual with a raw-count slope and offset scaling example.
- Scaling analog values Siemens
Official SIMATIC documentation for converting raw ranges to physical quantities.
- How to Program the Correct PLC Equation ISA
ISA article deriving a complete 4–20 mA raw-count scaling example.