How to Read a PLC Wiring Diagram: DI, DO, AI, AO, and Field Devices

A PLC wiring diagram is basically a map of how field devices connect to the PLC, and how the PLC sends commands back to equipment.

The reading logic is simple:
Find the signal. Follow the wire. Identify the terminal. Understand the function.

The basic signal path is:
field device → PLC input → PLC program → PLC output → field device action

This article explains how to read a PLC wiring diagram from a practical control panel perspective.

For a direct live example, please refer to this.

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PLC Wiring Diagram vs Power SLD

A PLC wiring diagram and a power SLD are not completely different in reading method.

Both drawings show connections. You still need to trace from one point to another and understand what each component does.

The difference is mainly the type of circuit.

In a power SLD, you usually trace the power path:

incoming supply → breaker → busbar → feeder → motor or load

In a PLC wiring diagram, you usually trace the control signal path:

field device → terminal block → PLC input

or:

PLC output → relay / VFD / servo drive → field device action

Drawing Type	What You Trace	Common Components	Main Question
Power SLD	Power path	Breakers, busbars, transformers, feeders, motors	Where does the power go?
PLC wiring diagram	Control signal path	PLC terminals, relays, sensors, VFD terminals, terminal blocks	What signal enters or leaves the PLC?

In both cases, the basic skill is the same:

follow the connection, identify the component, and understand the function.

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Basic Reading Method Before the Live Example

Click here to jump to the live examples.

Before reading a real PLC wiring diagram, use three basic steps.

1. Find the PLC Module

First, find the PLC or PLC I/O module on the drawing.

The PLC may be marked as:

• CPU
• PLC
• controller
• digital input/output module
• analog input/output module
• expansion I/O module
• communication module

The PLC module is usually drawn as a rectangle with many terminals.

Common terminal labels include:

Terminal Label	Meaning
X / DI	Digital input
Y / DO	Digital output
AI	Analog input
AO	Analog output
24V	Control power positive
0V	Control power common
COM	Common terminal
RS485 A/B	Communication terminal

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2. Do Not Read Only the Terminal Number

A terminal number only tells you where the wire is connected.

It does not fully explain the circuit.

For example, X6 only tells you the signal enters PLC input X6.

But if the drawing says:
X6 = Power Healthy
Now the signal has meaning.

The reading path may be:
24V → KM2 auxiliary contact → PLC input X6
If the KM2 contact closes, 24V reaches X6, and the PLC reads X6 as ON.

So the PLC program can understand:
X6 = ON → Power Healthy condition is true

The key point is:

Do not stop at the terminal number. Find the signal name and the device behind it.

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3. Follow Wire Numbers and Cross-References

One PLC signal may not stay on one page.

It may go through:

• wire number
• cross-reference
• relay contact
• terminal block
• connector
• another drawing sheet
• field device

So do not read the drawing only from left to right.

Read it by tracing the signal path.

For a PLC input, the path may be:
field device → connector → terminal block → PLC input

For a PLC output, the path may be:
PLC output → relay / drive → connector → field device

The rule is simple:

Find the signal, follow the wire, identify the terminal, understand the function.

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Live Example: Reading a Real PLC Wiring Drawing

In a real drawing set, the PLC page is only one part of the full system.

For example, one control panel drawing set may include separate pages for:

Sheet	What It Shows
Power Circuit	Main power wiring
Control Circuit	Control power and control devices
Servo Control	Servo drive, motor, encoder, brake, and control signals
PLC	PLC CPU, I/O terminals, HMI connection
Relay	Relay module and related control wiring
Loop Connector	Connector wiring between panel and field loop
Machine Connector	Machine-side sensors, switches, and wiring
Terminations	External connection points

This kind of drawing should be read by signal path, not only by page title.

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Example: Reading the PLC Page

In above drawing, it is module DELTA DVP12SA2. In the below section, we will show model DVP16SP.

Item	DELTA DVP12SA2	DVP16SP
Role	Main PLC CPU / controller	Expansion I/O module
Can run PLC program?	Yes	No
Function	Executes logic, communicates with HMI/servo/other devices, controls I/O	Adds more input/output terminals
Typical terminals	X inputs, Y outputs, RS485, high-speed / special terminals	Extra X inputs and Y outputs
Used for	Main control logic	More field signals when CPU I/O is not enough

Together, these sections show how the PLC page is built: main PLC CPU, expansion I/O module, HMI connection, power supply, inputs, and outputs.
As shown below:

To summarize how to read the PLC Wiring Diagram:
The first step is to identify input and output terminals.

Then read each signal as a path.
Example:
field signal → connector / terminal block → PLC X input
or:
PLC Y output → relay / drive / connector → field action

The PLC page tells you the I/O point.
It does not always show the full field device.
*So if the signal continues to another sheet, follow the wire number or cross-reference.

Live Detail: Reading Contactor Feedback as PLC Input

• At the top, there is a 24V control power line. At the bottom, the signals go into PLC input terminals such as X5, X6, and X7.
• In the middle, there are contacts marked with tags such as KM1 and KM2. These are contactor or relay contacts used to tell the PLC whether a condition is true.

Interpretation:

PLC Input	Signal Name	How to Read It
X5	Manual Brake Release Switch	A manual switch condition enters the PLC
X6	Power Healthy	A contact closes when power condition is healthy
X7	Brake Release FB	A feedback contact closes when brake release is confirmed

The working path is:
24V → contact / switch → PLC input X terminal → PLC program
For example, for Power Healthy:
24V → KM2 auxiliary contact → X6

If power healthy condition is good → KM2 contact closes
If the KM2 contact closes → 24V reaches PLC input X6 → The PLC then reads this input as ON.

So the PLC program can understand:
X6 = ON → power healthy condition is true
*This assumes the KM2 auxiliary contact is normally open and closes when the power healthy condition is active. If the contact is normally closed, or if the PLC program uses inverted logic, the meaning of X6 may be different.

For Brake Release FB:
24V → brake release feedback contact → PLC input X7

If the brake release condition is confirmed → the feedback contact closes.
When this contact closes → 24V reaches PLC input X7 → The PLC then reads X7 as ON.

So the PLC program can understand:
X7 = ON → Brake Release feedback is received

However, the exact meaning depends on where the feedback contact comes from. If it is an auxiliary contact of a relay or contactor, it only confirms that the relay/contact has operated. If it comes from the brake mechanism or a brake status sensor, it can more directly confirm the actual brake release condition.

The key of these feedback design is: A PLC output may command something to happen, but the PLC still needs input feedback to know whether it actually happened.

For example:

Command	Feedback	Meaning
Command ON	X7 ON	Brake release confirmed
Command ON	X7 OFF	Command sent, but no feedback
Command OFF	X7 ON	Abnormal feedback or wiring issue

This is the key idea behind many PLC control circuits.

The PLC is not only sending commands. It is also checking conditions before and after the command.

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How to Read Digital I/O and Analog I/O

Most PLC wiring signals fall into four groups:

Signal Type	Meaning	Direction
DI	Digital input	Field device → PLC
DO	Digital output	PLC → field device
AI	Analog input	Field device → PLC
AO	Analog output	PLC → field device

When reading any PLC I/O circuit, ask:

Question	Why It Matters
Is this DI, DO, AI, or AO?	Defines the signal type
Where does the signal start?	Finds the source
Where does the signal end?	Finds the destination
What terminal does it use?	Confirms the physical connection
What does the signal mean?	Connects wiring to control logic
Does it need common, 0V, or shield?	Confirms the complete circuit

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How to Read Digital Input Wiring

A digital input is a 1 or 0 signal entering the PLC.

It answers a yes/no question.

Field Question	PLC Signal
Is the start button pressed?	DI
Is the emergency stop released?	DI
Is the contactor closed?	DI
Has the overload tripped?	DI
Is the VFD in fault?	DI

The reading path is usually:

field contact → terminal block → PLC DI terminal → PLC program

Example:

contactor auxiliary contact → terminal block → PLC DI

This tells the PLC whether the contactor is actually closed.

When reading DI wiring, check:

Item	What to Check
Field device	Which contact or switch sends the signal?
Contact type	Normally open or normally closed?
Input voltage	Commonly 24 VDC, but must be confirmed
PLC terminal	Which DI terminal receives the signal?
Common / 0V	Is the input common wired correctly?
Signal meaning	Does DI = 1 mean normal, fault, open, closed, start, or stop?

Important point:

Do not only read “DI3.” Read what DI3 means.

For example:

DI3 = VFD fault is useful information.
DI3 alone is only a PLC address.

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How to Read Digital Output Wiring

A digital output is also a 1 or 0 signal, but it leaves the PLC.

It is a command.

PLC Command	Output Type
Start motor	DO
Energize relay	DO
Turn on alarm lamp	DO
Reset VFD fault	DO
Open solenoid valve	DO

The reading path is usually:

PLC DO terminal → relay / contactor / VFD terminal → field action

Example:

PLC DO → interface relay → contactor coil

This means the PLC does not directly power the motor. It sends a control command. The relay, contactor, VFD, or MCC circuit performs the actual switching.

When reading DO wiring, check:

Item	What to Check
PLC output terminal	Which DO terminal sends the command?
Output type	Relay output or transistor output?
Load device	Relay coil, contactor coil, VFD input, lamp, buzzer, solenoid
Control voltage	24 VDC, 110 VAC, 230 VAC, etc.
Protection	Fuse, diode, relay isolation, or interposing relay
Feedback	Is there a DI signal proving the command worked?

Important point:

A DO command does not prove the equipment actually operated.

Example:

PLC DO → contactor coil
contactor auxiliary contact → PLC DI feedback

Then the PLC can compare command and feedback:

DO Command	DI Feedback	Meaning
DO = 1	DI = 1	Command worked
DO = 1	DI = 0	Command sent, but no feedback
DO = 0	DI = 1	Abnormal feedback or wiring problem

This is a very common pattern in motor control panels.

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How to Read Analog Input Wiring

An analog input is a changing value entering the PLC.

It is not only 1 or 0.

Common analog input signals include:

• pressure
• temperature
• level
• flow
• pH
• current
• voltage
• speed feedback

The reading path is usually:

sensor / transmitter → terminal block → PLC AI terminal → PLC program

Example:

pressure transmitter → 4–20 mA → PLC AI

The PLC analog input module converts the analog signal into digital data. This is called A/D conversion.

The process is:

analog signal → AI module → digital numerical value → PLC program

Example:

Pressure	Signal	PLC Meaning
0 bar	4 mA	Minimum value
5 bar	12 mA	Middle value
10 bar	20 mA	Maximum value

This does not mean analog input becomes digital input.

A digital input is a physical ON/OFF terminal.

An analog input becomes a numerical value inside the PLC program.

When reading AI wiring, check:

Item	What to Check
Signal source	Which transmitter or sensor sends the value?
Signal type	4–20 mA, 0–10 V, RTD, thermocouple, etc.
PLC AI support	Does the analog module support this signal type?
Positive / negative	Are signal polarity and common correct?
Shielding	Is shielded cable required?
Scaling	What engineering value does the signal represent?

Example:

4–20 mA = 0–10 bar

If the PLC receives 12 mA, the program may read about 5 bar.

Then the PLC can create internal logic:

pressure < 3 bar → pressure low = TRUE

But “pressure low” is an internal logic result. It is not a physical DI terminal.

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How to Read Analog Output Wiring

An analog output is a changing command leaving the PLC.

It is commonly used for:

• VFD speed reference
• valve opening control
• damper position control
• pressure control
• flow control

The reading path is usually:

PLC AO terminal → field device analog input → controlled action

Example:

PLC AO → 0–10 V / 4–20 mA → VFD analog input

The PLC program calculates a command value first. Then the analog output module converts that internal value into a real analog signal. This is called D/A conversion.

The process is:

PLC program value → AO module → analog signal → VFD / valve / field device

Example for VFD speed control:

PLC Speed Command	AO Signal	VFD Meaning
0%	0 V / 4 mA	Minimum speed
50%	5 V / 12 mA	Medium speed
100%	10 V / 20 mA	Full speed

When reading AO wiring, check:

Item	What to Check
PLC AO terminal	Which AO channel sends the signal?
Signal type	0–10 V or 4–20 mA?
Field device input	Does the VFD or valve accept the same signal type?
Common terminal	Is analog common connected correctly?
Shielding	Is shielded cable used if required?
Parameter setting	Is the VFD or device configured for the same signal type?
Scaling	What does 0%, 50%, or 100% mean in the field device?

Important point:

Analog output wiring is not enough by itself. The field device parameter must also match the signal.

For example, if the PLC sends 4–20 mA, but the VFD is set to read 0–10 V, the speed command will not work correctly.

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What the Wiring Diagram Does Not Tell You

A wiring diagram tells you how devices are connected.

It does not fully tell you the PLC program logic.

From the wiring diagram, you can understand:

• which signal enters the PLC
• which output leaves the PLC
• which terminal is used
• which relay, connector, or field device is connected
• how the circuit is physically wired

But the wiring diagram usually does not show:

• ladder logic
• function block logic
• timers
• counters
• interlock sequence
• alarm rules
• servo motion sequence
• HMI screen logic

So the difference is:

Wiring diagram = how devices are connected
PLC program = when and why the PLC acts

To fully understand the control behavior, you need both the wiring diagram and the PLC program or control sequence description.

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Conclusion

For each signal, ask:

Is it DI, DO, AI, or AO?
Where does it start?
Where does it go?
What terminal does it use?
What does the signal mean?

A good PLC wiring diagram makes every signal easy to trace from the field device to the PLC, and from the PLC back to the controlled equipment.

FAQ

A PLC wiring diagram shows how field devices, PLC terminals, relays, connectors, sensors, drives, and control power are physically connected. It helps engineers trace signals entering and leaving the PLC.

DI, or digital input, is a 1/0 signal entering the PLC.
DO, or digital output, is a 1/0 command leaving the PLC.

AI, or analog input, is a changing value entering the PLC, such as pressure, level, temperature, or flow.
AO, or analog output, is a changing command leaving the PLC, such as a VFD speed reference.

No. Analog input becomes digital numerical data, not a physical digital input.

For example:
4–20 mA → A/D conversion → pressure value in PLC program

The PLC program may then compare this value with a setpoint and create an internal logic condition.