How-To Guide  ·  Allen-Bradley Analog I/O  ·  Signal Selection & Application

Analog Signal Types: 4–20 mA, 0–10 V, RTD, Thermocouple — When to Use Each

Platforms: Compact 5000 (5069), Compact I/O (1769), ControlLogix (1756)  ·  Studio 5000 Logix Designer

1. Analog vs. Digital Signals

A digital signal has two states: on (1) or off (0). A 24 V DC proximity sensor, for example, tells the PLC that a part is present or absent — nothing in between.

An analog signal carries a continuously variable value across a defined range. A pressure transmitter outputting 4–20 mA doesn’t just say “pressure exists” — it tells you the pressure is exactly 47.3 PSI. The PLC’s analog input module converts this electrical signal into a digital number (typically 16 bits, or 65,536 discrete values) that your program can use for control, alarming, trending, and data logging.

2. 4–20 mA Current Signals

The 4–20 mA current loop is the most widely used analog signal in industrial automation. It was introduced in the 1950s and remains the dominant standard today because of its exceptional noise immunity and built-in fault detection.

How It Works

A transmitter regulates current flow in a loop circuit. The current varies proportionally with the measured process variable:

• 4 mA = zero / minimum of range (e.g., 0 PSI)
• 20 mA = full scale / maximum of range (e.g., 100 PSI)
• 12 mA = midpoint (e.g., 50 PSI)

The “live zero” at 4 mA is what makes this signal type special: if the current drops to 0 mA, you know something is wrong — a broken wire, a dead transmitter, or a power failure. This built-in fault detection is impossible with 0–20 mA or voltage signals.

Wiring: 2-Wire vs. 4-Wire Transmitters

Typical Applications

• Pressure transmitters — tank pressure, pipeline pressure, differential pressure for flow
• Level transmitters — tank level via hydrostatic, ultrasonic, or radar
• Flow meters — magnetic flow meters, vortex, Coriolis
• Temperature transmitters — RTD or thermocouple with built-in transmitter
• Valve positioners — control valve feedback (position 0–100%)
• Analytical instruments — pH, conductivity, dissolved oxygen

Allen-Bradley Modules Supporting 4–20 mA Input

3. 0–20 mA Current Signals

The 0–20 mA range uses the same current-loop principle as 4–20 mA but starts at zero instead of 4 mA:

• 0 mA = zero / minimum of range
• 20 mA = full scale / maximum of range

Why Use 0–20 mA Instead of 4–20 mA?

0–20 mA is primarily found in older installations, laboratory instruments, and specialized equipment. For new designs, 4–20 mA is almost always the better choice due to built-in wire-break detection.

Typical Applications

• Laboratory instruments — analytical equipment, spectrometers
• Legacy process equipment — older transmitters and positioners
• Test & measurement — data acquisition systems

All Allen-Bradley analog input modules that accept 4–20 mA also accept 0–20 mA — you simply select the range in the channel configuration in Studio 5000.

4. Voltage Signals: 0–10 V, 0–5 V, 1–5 V

Voltage signals transmit a process value as a proportional voltage level. The transmitter or sensor outputs a voltage that the PLC’s analog input module reads directly.

Common Voltage Ranges

How Voltage Signals Work

The source device outputs a voltage proportional to the process value. The receiving module measures this voltage using a high-impedance input (typically >1 MΩ for the 1769-IF4, >1 MΩ for the 5069-IF8) so it draws negligible current from the source.

Voltage vs. Current: Key Differences

Typical Applications

• VFD speed reference — 0–10 V to set motor speed on drives like the PowerFlex 525
• HVAC systems — damper and valve actuators, building management systems
• Potentiometer position feedback — linear or rotary position sensing
• Weighing systems — load cells with voltage output amplifiers
• Short-distance panel-mount instruments — where wiring runs are under 30 m

The 1–5 V “Live Zero” Trick

You can convert any 4–20 mA signal to 1–5 V by placing a precision 250 Ω resistor across the input terminals. This is useful when you need to feed the same signal to both a PLC module and a panel meter or recorder. The 1 V live zero preserves the wire-break detection of 4–20 mA.

5. Bipolar Voltage: ±10 V

A ±10 V bipolar signal swings from −10V to +10V, passing through zero. This is essential for applications where the measured variable can be positive or negative.

How It Works

• −10V = full negative (e.g., full reverse speed)
• 0V = zero / neutral (e.g., stopped)
• +10V = full positive (e.g., full forward speed)

On a 16-bit module like the 5069-IF8, the ±10V range provides a resolution of approximately 320 µV per count (20V ÷ 65,536 counts), per the 5069-TD001 technical data publication.

Typical Applications

• Servo drive speed/torque reference — bidirectional speed commands
• Tension/compression load cells — force measurement in both directions
• Torque sensors — positive/negative torque measurement
• Vibration monitoring — accelerometer signals centered around zero
• Test equipment — signal generators, data acquisition systems

Module Support

Most Allen-Bradley voltage-capable modules support ±10V. On the Compact 5000 platform, both the 5069-IF8 and 5069-IF4IH accept ±10V inputs. On Compact I/O, the 1769-IF4, 1769-IF8, and 1769-IF16V all support ±10V.

6. RTD (Resistance Temperature Detectors)

An RTD is a temperature sensor that works on a simple principle: the electrical resistance of a metal changes predictably with temperature. As the temperature increases, the resistance of the metal element increases in a nearly linear relationship.

How RTDs Work

The PLC module sends a small excitation current (typically 0.5–1.0 mA) through the RTD element and measures the resulting voltage. Using Ohm’s Law (R = V/I), it calculates the resistance, then converts that to temperature using a standard curve (e.g., IEC 60751 for platinum RTDs).

Common RTD Types

Temperature ranges and RTD types from Rockwell Automation publication 5069-TD001 and 1769-TD006.

Wiring Configurations

Typical Applications

• Process temperature measurement — reactors, heat exchangers, ovens, tanks
• HVAC — duct and pipe temperature sensing
• Clean rooms & pharma — precision temperature monitoring
• Motor winding temperature — embedded Cu10 or Pt100 RTDs
• Food & beverage — pasteurization, cold chain monitoring

Allen-Bradley RTD Modules

RTD accuracy and supported types from Rockwell Automation publications 5069-TD001 (page 29–30) and 1769-TD006 (page 42–44).

7. Thermocouple Signals

A thermocouple is a temperature sensor made from two dissimilar metals joined at one end. When the junction is heated, it produces a small voltage (millivolts) proportional to the temperature difference between the junction and the reference point. This is known as the Seebeck effect.

How Thermocouples Work

1. Two different metal wires are welded together at the measurement junction (hot junction).
2. The other ends connect to the PLC module’s terminals — the reference junction (cold junction).
3. The temperature difference between the two junctions produces a small voltage (typically 0–75 mV).
4. The module’s Cold Junction Compensation (CJC) sensor measures the reference junction temperature and compensates mathematically.
5. The module converts the compensated millivolt signal to a temperature reading using standard linearization tables (NIST ITS-90).

Common Thermocouple Types

Thermocouple types and ranges from Rockwell Automation publications 5069-TD001 (page 30) and 1769-TD006 (page 45).

RTD vs. Thermocouple: When to Choose Each

Allen-Bradley Thermocouple Modules

Thermocouple accuracy from Rockwell Automation publications 5069-TD001 (page 30) and 1769-TD006 (page 45–46).

8. Signal Type Comparison

The table below summarizes all six signal types to help you choose the right one for your application.

9. Allen-Bradley Analog Module Selector

Use this table to find the right module for your signal type and platform.

Analog Input Modules

Analog Output Modules

10. Scaling Analog Signals in Studio 5000

Most Compact 5000 (5069) modules output data in engineering units — the tag value is already in volts or milliamps. Compact I/O (1769) modules can be configured for engineering units, scaled-for-PID, percent of range, or raw/proportional data.

Compact 5000 Modules (5069)

The 5069-IF8 and other 5069 analog modules report data in IEEE 754 32-bit floating-point format (REAL data type). If a channel is configured for 4–20 mA, the tag reads directly in milliamps (4.000 to 20.000). You scale from milliamps to engineering units using the SCP (Scale with Parameters) instruction:

// Scale 4-20 mA pressure transmitter (0-100 PSI)
SCP  Input:       Local:1:I.Ch0Data    // Raw mA value (4.0 - 20.0)
     InputMin:    4.0                   // 4 mA = 0 PSI
     InputMax:    20.0                  // 20 mA = 100 PSI
     OutputMin:   0.0                   // 0 PSI
     OutputMax:   100.0                 // 100 PSI
     Output:      Pressure_PSI         // Scaled result

Compact I/O Modules (1769)

The 1769 analog modules offer four data formats. Set the format in the module’s Channel Configuration in RSLogix 5000 / Studio 5000:

Scaling Formula

The general formula for linear scaling is:

Engineering Value = ((Raw − RawMin) ÷ (RawMax − RawMin)) × (EngMax − EngMin) + EngMin

11. Wiring & Noise Best Practices

1. Use shielded cable for all analog signals. Belden 8761 (for current/voltage) or Belden 9501/9533 (for RTD) are industry standards referenced in the Rockwell technical data publications.
2. Ground the shield at one end only — at the module end. This prevents ground loops that create noise. The 5069 analog modules have dedicated shield terminals on the RTB for this purpose.
3. Separate analog and digital wiring in different conduit or cable tray. Keep analog cables at least 12 inches from power wiring and VFD output cables.
4. Use twisted-pair cable to cancel electromagnetic interference. Each signal pair should be individually twisted.
5. Use 4–20 mA for long runs — voltage signals degrade with distance. If you must use voltage over a long run, use heavier gauge wire to reduce resistance.
6. Use differential inputs when available. The 5069-IF8 Series A provides 8 differential inputs; the 1769-IF4 supports both differential and single-ended wiring.
7. Configure input filters to reject power-line noise. Set the notch filter to 50 or 60 Hz (matching your local mains frequency) to reject the most common source of analog noise.
8. Keep RTD excitation current low (0.5 mA rather than 1.0 mA) to minimize self-heating error, especially with Pt100 elements.

12. Related Guides & Resources

Related PLC Exchange How-To Guides

Rockwell Automation Reference Documentation