3. Instrument Signals: How Field Devices Talk to the PLC

May 25, 2026 JoeGuardian 0

Discrete, Analog, and Digital Signals for Automation Technicians

Industrial instruments measure real-world process conditions such as pressure, level, flow, temperature, weight, pH, or gas concentration. But the PLC cannot directly read those physical conditions.

The PLC needs a signal.

A signal is the way a field device communicates information to the control system.

In simple terms:

An instrument signal is the language used between the field device and the PLC, controller, HMI, or SCADA system.

A pressure transmitter may measure pressure, but the PLC does not directly “see” pressure. The PLC sees a current, voltage, discrete input, pulse, or digital communication value that represents that pressure.

The book Lessons In Industrial Instrumentation covers multiple signal and communication methods used in industry, including discrete I/O, analog I/O, 4–20 mA current loops, HART, Modbus, Ethernet networks, and PLC input/output systems.

Why Instrument Signals Matter

For an automation technician, understanding signals is critical because many troubleshooting problems are not caused by the PLC program itself.

The issue may be:

A broken wire
A missing 24 VDC supply
A failed transmitter
A bad analog loop
Wrong scaling
Electrical noise
A misconfigured input card
A communication problem
A bad HMI tag

A good technician knows how to follow the signal path:

Process → Instrument → Signal → PLC Input → Logic → HMI Display

If the HMI value is wrong, the technician must determine whether the problem is in the process, the instrument, the signal, the PLC, the scaling, or the HMI.

The Three Main Types of Instrument Signals

Most field signals can be grouped into three major categories:

Signal Type	Basic Meaning	Common Example
Discrete	ON/OFF signal	Pressure switch, level switch, proximity sensor
Analog	Variable signal	4–20 mA pressure transmitter
Digital Communication	Data over a protocol	HART, Modbus, Ethernet/IP

Each type has a different purpose and troubleshooting method.

1. Discrete Signals

A discrete signal has only two states:

ON or OFF
TRUE or FALSE
1 or 0
Energized or De-energized

Discrete signals are used when the PLC only needs to know whether something is present, active, safe, open, closed, high, low, running, or faulted.

Common Discrete Devices

Device	Example PLC Meaning
Pressure Switch	Air pressure OK
Level Switch	Tank low level
Flow Switch	Flow proven
Temperature Switch	High temperature trip
Limit Switch	Door fully open
Proximity Sensor	Part detected
Motor Starter Auxiliary Contact	Motor running feedback
Overload Contact	Motor overload tripped

Example: Pressure Switch

A pressure switch may be wired to a PLC discrete input.

Air Pressure OK Switch → PLC Digital Input

The PLC logic may use it like this:

IF Air_Pressure_OK = TRUE
THEN Machine_Start_Allowed = TRUE

This type of signal is simple, but very important.

A discrete signal is commonly used for:

Permissives
Interlocks
Alarms
Faults
Position feedback
Start/stop logic
Safety monitoring
Machine status

Normally Open and Normally Closed Signals

Automation technicians must understand the difference between the field device contact and the PLC logic condition.

A field device may be wired as:

Contact Type	Meaning
Normally Open	Contact closes when the condition is active
Normally Closed	Contact opens when the condition is active

But in the PLC, the logic may be written based on the desired condition:

DI_Air_Pressure_OK
DI_Guard_Closed
DI_Motor_Overload_OK
DI_EStop_OK

This is why naming matters.

A good tag name should describe the healthy or useful condition, not just the electrical contact.

Example:

Good: DI_Air_Pressure_OK
Less Clear: DI_Pressure_Switch

2. Analog Signals

An analog signal changes continuously over a range.

Instead of being only ON or OFF, it represents a process value.

Example:

4–20 mA = 0–100 PSI

Analog signals are used when the PLC needs to know how much pressure, level, flow, temperature, weight, or pH exists.

Common Analog Instruments

Instrument	Common Signal
Pressure Transmitter	4–20 mA
Level Transmitter	4–20 mA
Flow Transmitter	4–20 mA or pulse
Temperature Transmitter	4–20 mA
pH Transmitter	4–20 mA
Weight Transmitter	4–20 mA or digital
Control Valve Positioner	4–20 mA command

4–20 mA: The Most Common Analog Signal

The 4–20 mA current loop is one of the most common industrial instrumentation signals.

A typical relationship looks like this:

Signal	Process Value
4 mA	0%
12 mA	50%
20 mA	100%

Example:

0 PSI   = 4 mA
50 PSI  = 12 mA
100 PSI = 20 mA

This is useful because the PLC can scale the analog input into engineering units.

4–20 mA → PLC Analog Input → Scaling → PSI, GPM, °F, %, gallons

Why 4 mA Is Not the Same as 0 mA

This is a very important concept.

In a 4–20 mA signal:

4 mA = 0% process value
0 mA = bad signal condition

A 4 mA signal usually means the process is at the low end of the calibrated range.

A 0 mA signal may mean:

Open circuit
Broken wire
Lost power supply
Failed transmitter
Disconnected loop
Bad analog input connection

This is called a live zero signal.

The signal starts at 4 mA instead of 0 mA so the control system can distinguish between a valid low reading and a failed loop.

Example: Level Transmitter

A level transmitter may be ranged like this:

0% tank level = 4 mA
50% tank level = 12 mA
100% tank level = 20 mA

If the HMI shows 0%, the technician should not immediately assume the tank is empty.

The technician should verify:

Is the tank actually empty?
Is the transmitter outputting 4 mA?
Is the PLC analog input reading correctly?
Is the scaling correct?
Is the HMI tag correct?

This is professional troubleshooting.

0–10 VDC Signals

Another analog signal is 0–10 VDC.

This signal is common in some control systems, drives, HVAC equipment, and smaller automation systems.

Example:

0 VDC = 0%
5 VDC = 50%
10 VDC = 100%

However, 0–10 VDC signals are generally more sensitive to voltage drop and electrical noise than 4–20 mA loops, especially over long cable runs.

For process instrumentation, 4–20 mA is usually more common.

RTD and Thermocouple Signals

Temperature measurement may use special sensor signals before they are converted into a standard PLC signal.

RTD

An RTD changes resistance as temperature changes.

Example:

Temperature changes → Resistance changes

RTDs are commonly used when accuracy and stability are important.

Thermocouple

A thermocouple generates a very small voltage based on temperature difference.

Example:

Temperature difference → Small millivolt signal

Thermocouples are often used in high-temperature or rugged applications.

In many industrial systems, the RTD or thermocouple connects to a temperature transmitter. The transmitter then sends a 4–20 mA signal to the PLC.

RTD / Thermocouple → Temperature Transmitter → 4–20 mA → PLC Analog Input

Pulse Signals

Some instruments send pulse signals instead of a standard analog signal.

Pulse signals are common with:

Flowmeters
Encoders
Speed sensors
Totalizers
Metering systems

Example:

Each pulse = a known amount of flow

The PLC may count pulses to calculate:

Total gallons
Flow rate
Speed
Position
Production count

Example:

100 pulses = 1 gallon

Pulse signals are very useful, but the PLC input must be fast enough to capture them correctly.

3. Digital Communication Signals

Some modern instruments communicate digitally using industrial protocols.

Instead of sending only one analog value, a smart device can send multiple pieces of data.

Common Digital Communication Methods

Communication Type	Common Use
HART	Smart transmitters over 4–20 mA loops
Modbus RTU	Meters, analyzers, drives, instruments
Modbus TCP	Ethernet-based industrial devices
Ethernet/IP	PLCs, remote I/O, drives, smart devices
Foundation Fieldbus	Process instrumentation networks
Profibus PA / DP	Industrial field communication

HART Communication

HART is important because it allows digital communication over a standard 4–20 mA signal.

A HART transmitter can send:

Process variable
Device status
Diagnostics
Range values
Engineering units
Calibration information
Configuration data

The analog 4–20 mA signal may still represent the primary process variable, while HART carries additional digital information.

Example:

4–20 mA = Pressure value
HART = Device configuration and diagnostics

Modbus Communication

Modbus is commonly used with:

Power meters
Flowmeters
Analyzers
VFDs
Remote I/O
Temperature controllers
Weighing systems

Instead of reading one signal wire as ON/OFF or 4–20 mA, the PLC reads data from registers.

Example:

Modbus Register 40001 = Flow Rate
Modbus Register 40002 = Totalized Flow
Modbus Register 40003 = Device Status

With Modbus, troubleshooting includes:

Device address
Baud rate
Parity
Wiring polarity
Register address
Data type
Word order
Communication timeout

Ethernet-Based Instrument Signals

Some modern devices communicate over industrial Ethernet.

Examples:

Ethernet/IP instruments
Remote I/O blocks
Smart sensors
VFDs
Flowmeters
Gateways
Weighing systems
Vision systems

Ethernet-based devices may provide:

Process values
Diagnostic bits
Alarm status
Device health
Configuration parameters
Control commands

This type of communication is powerful, but it also requires the technician to understand basic networking:

IP address
Subnet mask
Gateway
Ethernet cable
Switch port
Link status
Device status LEDs
PLC communication path

Signal Type Comparison

Signal Type	PLC Reads	Example	Technician Checks
Discrete	ON/OFF	Pressure switch	24 VDC, input LED, contact state
Analog	Variable value	4–20 mA transmitter	Loop current, power, scaling
Pulse	Count/frequency	Flowmeter pulse output	Pulse rate, input speed, wiring
Digital	Data/registers	Modbus flowmeter	Address, protocol, data type
HART	Digital data over analog loop	Smart transmitter	Loop resistor, communicator, device status
Ethernet	Network data	Smart instrument	IP, cable, switch, PLC connection

How the PLC Uses Instrument Signals

Instrument signals are used in many ways inside the PLC.

Discrete Example

DI_Flow_Proven = TRUE

Used for:

Pump_Running AND NOT Flow_Proven → No_Flow_Fault

Analog Example

AI_Tank_Level_Pct = 65.3%

Used for:

IF Tank_Level_Pct < 15%
THEN Low_Level_Alarm = TRUE

Digital Communication Example

Flowmeter.Status = OK
Flowmeter.FlowRate = 45.2 GPM
Flowmeter.Total = 12500 gallons

Used for:

HMI display
Batch total
Production report
Alarm logic
Maintenance diagnostics

Common Signal Problems

Instrumentation signal problems are very common in real plants.

Discrete Signal Problems

Possible issues:

No 24 VDC supply
Broken wire
Loose terminal
Bad sensor
Wrong sensor alignment
Failed input card
Incorrect NO/NC wiring
Bad input common
Field device not actuated

Analog Signal Problems

Possible issues:

Open loop
Short circuit
Bad transmitter
Wrong range
No loop power
Bad analog input channel
Wrong scaling
Electrical noise
Poor shield grounding
Signal out of range

Digital Communication Problems

Possible issues:

Wrong IP address
Wrong node address
Bad cable
Bad connector
Incorrect protocol settings
Termination problem
Wrong register address
Timeout
Data type mismatch
Device not configured in PLC

Technician Troubleshooting Mindset

When troubleshooting any instrument signal, avoid guessing.

Use a step-by-step method.

Step 1 — Verify the Process

Ask:

Is the real process condition actually what the HMI says?

Example:

If the HMI shows low level, check the actual tank, sight glass, local indicator, or field gauge when available.

Step 2 — Verify the Instrument

Ask:

Is the instrument powered, installed correctly, and responding to the process?

Check:

Display
Status LEDs
Wiring
Process connection
Isolation valves
Sensor condition
Device health

Step 3 — Verify the Signal

For discrete:

Measure 24 VDC at the input.

For analog:

Measure loop current.

For digital:

Check communication status.

Step 4 — Verify the PLC Input

Check:

Input LED
Raw analog count
Module status
Channel status
Controller tag
Communication status

Step 5 — Verify Scaling and Logic

Ask:

Is the PLC converting the raw signal correctly?

Check:

Raw minimum
Raw maximum
Engineering minimum
Engineering maximum
Units
Alarm limits
Fault limits
HMI tag

Practical Example: Pressure Transmitter Signal

Instrument:

PT-101
Range: 0–100 PSI
Output: 4–20 mA
PLC Tag: AI_PT101_Pressure_PSI

Expected signal:

Pressure	Signal
0 PSI	4 mA
25 PSI	8 mA
50 PSI	12 mA
75 PSI	16 mA
100 PSI	20 mA

If the pressure gauge shows 50 PSI but the HMI shows 0 PSI, the technician should check:

Does the transmitter display show 50 PSI?
Is the loop current about 12 mA?
Is the PLC raw input changing?
Is the analog card configured correctly?
Is the scaling correct?
Is the HMI reading the correct PLC tag?

This method avoids replacing parts unnecessarily.

Practical Example: Flow Switch Signal

Instrument:

FSL-201
Flow Switch Low
Output: 24 VDC discrete input
PLC Tag: DI_Flow_Proven

PLC logic:

IF Pump_Running = TRUE
AND DI_Flow_Proven = FALSE
FOR 10 seconds
THEN No_Flow_Fault = TRUE

If the pump is running but the PLC does not see flow, check:

Is flow actually present?
Is the valve open?
Is the line blocked?
Is the flow switch adjusted correctly?
Is 24 VDC present?
Is the PLC input LED ON?
Is the PLC tag changing?
Is the timer logic correct?

Best Practices for Instrument Signals

1. Use Clear Tag Names

Good examples:

DI_Air_Pressure_OK
DI_Flow_Proven
AI_Tank_Level_Pct
AI_Discharge_Pressure_PSI
AI_Product_Temperature_F

Avoid unclear names:

Input_1
Sensor_A
Pressure
Analog_3

2. Separate Raw Inputs from Validated Signals

A professional PLC program should not use raw signals directly everywhere.

Better structure:

Raw Input → Input Buffer → Validated Signal → Logic

Example:

Local:1:I.Data.0 → DI_Raw_FlowSwitch → DI_Flow_Proven

For analog:

Raw Analog Count → Scaled Engineering Value → Validated Process Value

3. Add Signal Fault Detection

Analog signals should have fault detection.

Example:

If signal < 3.6 mA → Bad Signal / Under-range
If signal > 21 mA → Bad Signal / Over-range

This helps the PLC detect signal problems instead of treating them as valid process values.

4. Document the Signal Path

For every important instrument, document:

Instrument tag
Signal type
Range
PLC input card
Channel
Engineering units
HMI tag
Alarm setpoints
Calibration information

This makes troubleshooting much faster.

Key Takeaway

Industrial instruments communicate with the PLC through signals.

The main signal types are:

Discrete → ON/OFF condition
Analog → Variable process value
Digital → Data communication

A good automation technician understands not only what the instrument measures, but also how that measurement travels to the PLC and how the PLC uses it.

The professional troubleshooting mindset is:

Do not guess. Follow the signal.

Final Thoughts

Instrument signals are the connection between the real world and the automation system.

A sensor or transmitter measures the process. The signal carries that measurement to the PLC. The PLC processes the value, applies logic, triggers alarms, and displays information on the HMI.

Once you understand discrete, analog, and digital signals, industrial instrumentation becomes much easier to troubleshoot.

The next topic in this series will focus on one of the most important industrial signals:

4–20 mA current loops

Understanding 4–20 mA is essential for any automation technician working with pressure, level, flow, temperature, pH, weight, and control valves.

3. Instrument Signals: How Field Devices Talk to the PLC

Discrete, Analog, and Digital Signals for Automation Technicians

Why Instrument Signals Matter

The Three Main Types of Instrument Signals

1. Discrete Signals

Common Discrete Devices

Example: Pressure Switch

Normally Open and Normally Closed Signals

2. Analog Signals

Common Analog Instruments

4–20 mA: The Most Common Analog Signal

Why 4 mA Is Not the Same as 0 mA

Example: Level Transmitter

0–10 VDC Signals

RTD and Thermocouple Signals

RTD

Thermocouple

Pulse Signals

3. Digital Communication Signals

Common Digital Communication Methods

HART Communication

Modbus Communication

Ethernet-Based Instrument Signals

Signal Type Comparison

How the PLC Uses Instrument Signals

Discrete Example

Analog Example

Digital Communication Example

Common Signal Problems

Discrete Signal Problems

Analog Signal Problems

Digital Communication Problems

Technician Troubleshooting Mindset

Step 1 — Verify the Process

Step 2 — Verify the Instrument

Step 3 — Verify the Signal

Step 4 — Verify the PLC Input

Step 5 — Verify Scaling and Logic

Practical Example: Pressure Transmitter Signal

Practical Example: Flow Switch Signal

Best Practices for Instrument Signals

1. Use Clear Tag Names

2. Separate Raw Inputs from Validated Signals

3. Add Signal Fault Detection

4. Document the Signal Path

Key Takeaway

Final Thoughts

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